Suggest Treatment For Pain And Stiffness In Leg

Hi. Two days ago while I was playing sport I got really bad cramps in my legs, it started behind my knee and would gradually go up through both my legs. The day after, and today (two days later), it is sore at my lower hamstrings. I have tried stretching it lots but it doesnt seem to relieve it. If I sit for a while it gets stiff and returns to the same pain again. This pain is not cramps though. Just wondering what the best thing is to get rid of this pain. Thanks.