Go to your Primary Care doctor and discuss this problem. My insurance requires me to start here. It may be something they can treat or it may be something for a specialist. They may refer you to either a GYN, an Orthopedic Surgeon or possibly a GI (gastroenterologist).
Who they refer you to depends on the information you furnish them. You will be asked to describe the pain, when it started, what makes it better, what makes it worse, is it related to you monthly period, etc. Your doctor should manipulate your body or ask you to stand or bend over so they can see how your body is moving. Your primary may even order some lab work.
Talk to your primary to get the ball rolling. Good luck and write and tell us what you find out.