There isn't enough information in the question to really give a good answer.  A lot of pelvic pain can radiate down the legs, just because of the way the nerves are.  I have had menstrual cramps that were that bad, and currently whatever is going on with me is also causing the same problem when it's really bad (plus others).  Things that have been investigated or suggested as we've been looking for a diagnosis include ovarian cysts, endometriosis, and IBS... 

If you google pelvic pain and begin researching you'll discover probably a dozen possible causes.  Keep a log of when the pain is worst, what might make it worse, etc., and be sure to share that with your doctor.