I am really sore just under my right knee. Can anyone please tell me what is wrong with my leg?

It's located under my knee cap on the inside side of my right leg. I haven't done any over exercising or anything. I felt it a bit on tuesday night, but it disappeared before i had netball and tennis on wednesday. It then came back on Thursday morn for my athletics carnival. (along with very sore quads.) it's now Saturday and i can hardly walk on my right leg. Everytime I straighten my leg and put my weight on it (55kg) it absolutely kills me. Can anyone please help me? Should I go to the Physio? It has happened before and just went away but this time it is much worse. please help me!