Hi,
I seem to be suffering something similar - the lymph nodes in my groin become painful, I can become feverish and then it hits the next day spreading across my left buttock and hip and (once) right down the back of my left leg, covering a reasonably large, and worrying, percentage of my body.
It took me a while to get the doctor to agree with me, as, although it can take a week or two to go, I can usually be over the worst of it in a few days and so the doctor initially thought it was a rash as I didn't get to him quick enough. The second time it occured was on Christmas Day and couldn't get an appointment for days so gave up. With, the most recent outbreak I got there in time and he: a) agreed it was cellulitis (I'd had it once before elsewhere, not connected with this, so knew what it was when I saw it), b) he'd never seen anything like it and c) he was mystified as to the cause. I've also had numerous small infections that looked like they were about to go full-blown but I managed to fight it off, with only a small patch of reddish skin, after the initial symproms.
Last time I had an extra symptom - it came on hard when I was asleep with none of the usual early warning signs and was pretty much full-blown when I awoke, in addition I felt wretched (the kind of ache you get with, for example, flu or food poisoning), confused and light-headed. I collapsed unconcious once and was all over the place, nearly going flat out on the floor a second time before I crawled to the couch and waited it out. I eventually recovered after 3-4 hours but it was a worrying development.
As the doctor didn't know what it was he ordered a panel of blood tests, which all came back normal - kidney and liver function, white blood cell count, inflammatory markers, etc. so he didn't know where to go.
Luckily, I did some research online, finding this discussion and another one (this also seems to include some unrelated problems too, but includes other people who sound like they also have something similar - the original poster doesn't reporting recurring issues, although they may have amputated the buttock before it could, but indiyanna, Piperlady and neenee1 do):
http://www.medhelp.org/posts/Rare-Diseases/Cellulitis-Buttocks-SURGERY-REMOVAL-plz-answer/show/655696
Unfortunately, neither this thread or the other have any answers, which is a worry.
However, I did dig up some other information:
http://www.ncbi.nlm.nih.gov/pubmed/21597579
http://www.ncbi.nlm.nih.gov/pubmed/11578067
Both suggest that atypical cellulitis of the buttocks can be caused by a psoas abcess:
https://en.wikipedia.org/wiki/Psoas_muscle_abscess
http://www.mdguidelines.com/abscess-psoas
This can present in a number of ways depending on which direction it empties out, cellutis being one of the signs if it empties towards the buttock/hip. While antibiotics can clear up the cellulitis, the abcess needs to be located, drained and then treated with hefty antibiotics or it seems obvious that it can happen again and again.
The worry about that, beyond the fact that it is usually caused by something else going wrong somewhere in the general region (it was lumbar TB and so was a lot more common back in the day, not it is Crohn's - as I say I had no inflammatory markers, so this seems unlikely, the tumour described in the second abstract would show up with cellulitis on the right side), is that untreated it is always fatal and it kills you by sepsis, the symptoms of which sound very much like what I had in addition to the cellulitis. I am generally healthy at the moment so survived, but if I hadn't been... Anyway, at least next time I'll take it seriously (I feel like I dodged a bullet there, so won't let that happe again) and go straight to hospital, if I can concentrate enough to work out I need to, a nasty Catch 22 right there.
The doctor wasn't 100% convinced (he suggested it is was unlikely to go away if it was an abcess, although the second abstract I link to says they treated the cellulitis and then addressed the abcess months later), but he liked it enough to refer me to a colo-rectal surgeon who will be able to look into this further and suggested they'd think he was a "clever doctor" for suggesting such a rare potential source of the problem (he has credited me in the results, so hasn't stolen all the "glory" for himself 
). He also suggested that the cellulitis itself might have been sufficient to cause the sepsis. Luckily, it was enough to kick this up a rung of the ladder to a specialist who might stand a better chance of working out what is going wrong, even if my initial diagnosis doesn't pan out.
Now it might not be an abcess, but it is certainly worth checking out if nothing obvious shows up, like a compromised immune system. The problem I see for you missrachael is that the ultrasound tests (and perhaps the MRI) should have shown it up, although I will be asking for a CT scan, which sounds like the best way of finding one. It is worth mentioning to your doctor anyway, so they can look for it specifically, if they haven't already.
Anyway, that is the story so far. My appointment is still a couple of months away, I'll see how it goes. Fingers crossed that, between us we can figure out what the underlying issue might be, although I'm also resigned to the possibility that there may be more than one cause, so one fix might not solve every problem - it won't stop me digging though, this is just my best first shot. Let us know how things go, for you and everyone suffering.
Good luck.
However, I am still hopeful that someone will come up with something eventually, even if it involves more tests.