Is the pain localized or is it spread throughout the whole thigh?

If it is localized, it could be a "micro-tear" (man, I'm having a lot of trouble translating medical terms, I'm sorry) of your muscle, and you need to cease sporting activities immediately.

If it is a general ache, you should rest and put a hot compress for the muscle to relax. Also some of those medicinal rubs for athletes, (read: methyl salicylate or ibuprofen [I had to look those up in English you bastard, it took a while

]) will help.
Was it playing basketball or soccer?
EDIT: 100 posts / 3 pages of my thread! Yay! I'm excited!
