Pregnant on the Depo Injection after 6 years??

I've been on the depo injection for 6 years now, since I was 16 and i'm now 22.

I have had no issues with the form of contraception until recently when I felt pregnant. I am sexually active and don't use any other form of contraception (condoms etc.) It's just the 6 years i've been on it, I haven't felt like this before. My periods stopped pretty much straight away when I got on the injection so I haven't had any bleeding or a full period in 6 years! The symptoms i'm now suddenly experiencing are:

-Pelvic cramps

-Stomach bloating and aching 

-Nausea 

-Extreme Fatigue

-Mood Swings

-No enegery

I thought I could be pregnant but it's so rare and especially because i've been on it so long, I took a home pregnancy test and it was negative but I still feel really pregnant. 

I was just wondering if anyone else on it has felt the same or has got pregnant on it, any advice would be appreciated.