Hi Juke,
I realise that this was posted over a year ago, but I needed to reply because I have the exact same thing.
Just over three years ago (aged 19), I was also working in retail and I woke up at 4am with agonising hip pain. After calling 111, I was taken to A&E by ambulance (I was given Gas & Air and this didn't touch the pain at all). Doctor after doctor came into my room, each one gave me a tablet for pain relief and left after asking the usual questions. By the fourth doctor, and fourth pain relief tablet, my pain had obviously subsided to about a 4/10 so I was sent home. I was unable to walk the first couple of days and had a limp for the next couple of weeks, but then it disappeared and I thought it was a freak incident. Bear in mind, that I wasn't regular exercising at this point due to the amount of physcial activity in my job.
Fast forward six months and it happens all over again, but by the time I could get to the doctors (2-3 days) the pain was halved and I was merely limping, so again they gave me pain relief (Cocodamol 8mg) and sent on my way.
After that, it kept happening every couple of months. Eventually, I moved up from Cocodamol 8mg to 30mg, then that stopped working so I'm currently on Tramadol 50mg.
At first, I was told it was a groin sprain and sent to physio therapy but by the time I got there (3-4 weeks) the pain had virtually disappeared. So physio discharged me.
Next, I was referred to hospital and had an X-Ray. Nothing shows up, so there isn't a problem with the bone.
Then, I had an Arthrogram and MRI. I was told I had a suspected Labral Tear in my right hip, however nothing was wrong with my left hip. I was given Cortisone injections in both hips to see if this would remove the pain. Unfortunately, this was unsuccessful so I was referred to a doctor who specialised in hips. He performs another Arthrogram and diagnoses that there is actually nothing wrong with either of my hips. Not a Labral Tear, no FAI, nothing. He actually said that the movement was very good in my hips, so I now know that it's not the Labrum.
Following this news, I was told I would just be referred to a pain clinic. The doctor said that it could be ligament damage, but I doubt that the pain would be so intermittent if this were the case.
Please let me know if you are any further with a diagnosis. Unfortunately, I have no advice because there is no trigger for the pain in my case and the only way to relieve it is by taking Tramadol 50mg. I have heard exercise is good, but I only do yoga because I'm scared to death of exacerbating the pain.