What Is The Treatment For Severe Knee Pain?

My left knee hurts and is sore to the touch below the knee cap on the inside of the knee. I play tennis regularly but never felt an injury occur. Although it doesn't really appear to be swollen (maybe a smidge), it feels very thick and range of motion is limited without increasing pain. Neither ice nor heat seem to help. This has been going on for a solid two weeks. I did not play tennis for about a week and a half and the knee got better but not healed. Played tennis yesterday and it hurts now as much as it ever did early on (but probably not more than). Thanks. Jill