Pelvic girdle pain post partum

Hello. I had my baby via c section 4 weeks ago (second child). I suffered with pelvic girdle pain during pregnancy but it went away when baby was born. However, last night I slept on my side for the first time since c section (with pillow between my legs) and I woke up with really bad pelvic girdle pain! Why has it come back? Is it because I slept on my side? Should I sleep on my back again now? Is this normal? Of anyone else knows a lot about pgp please let me know... 4 weeks post section I was starting to feel much better but this has really set me back! Thanks.