Owies...well I can think of all sorts of possible causes, but perhaps get to a doc/physio/one-of-those-types-of-people and see what it is and what they can do for you..

I could speculate for ages, but I don't know what's wrong, so yeah, go and ask a pro what you can do to relieve the pain is my advice.
All the best, hope it stops hurting soon!