Pain under knee caps

You probably have jumpers knee. I got one of these things at Sports Authority because I had your problem too.
http://www.sportsauthority.com/product/index.jsp?productId=1300110&view=grid

Yeah I have a huge problem with Patellar tendinitis (Jumper's Knee). I re-aggravate my left leg very often.
http://www.massgeneral.org/ortho/services/sports/injuries_patellatendinitis.aspx

It's called Pateller Tendinitis. It's pretty common issue for athletes, specifically long jumpers and football players. Obviously skiers can get it as well, if youve been taking landings to flat lately thatll do it.

Don't bother with one of those knee bands unless its to the point you have trouble walking or the following suggestion isnt available to you. You get some pre-wrap

for cheap and then literally just tie a piece around your leg and roll it into a thin band and place it at the lowest part of your patella. itll take care of it. If it is more serious you can also find some hard circular metal or pvc piping, (i used the lower framing of a soccer goal during track with the same issue), position your lower patella on it and move slowly and firmly forward and backward.