Today While Walking I Suddenly Had A Sharp Pain In

Today while walking I suddenly had a sharp pain in my knee. It has ached for 3 hours since. I am most comfortable with it flexed. I can't straighten it. I have pain on the inside of the knee, the outside and behind the knee. When I walk I avoid straightening it as i normally would. This is causing a limp. Sometimes the sharp pain returns when I walk but most of the time it simply aches. I don't see any swelling or discoloration.