Severe Pelvic Pain

Hello, girl, I am 18. I have had a terrible pelvic pain for about a year, but it only appears when I sleep on my right side with a bent knee. My leg hurts and in the evening I wake up from a severe pain in my pelvis. I can barely move my legs, after moving it it cracks and the pain stops. It also happens when I feel cramped and immobile for a long time. What could be the reason for this and what kind of doctor should I visit?