Back of left knee

I noticed there is a tender spot at back of my knee. It is like a jelly inside. It is painful when I am waking. I feel like my knee is going to give way. I noticed too by the time i reach the 4th floor climbing the stairs, i have pain on my left and right knee. I do a lot of sports such biking, basketball but mostly playing soccer. What do you think I have on my left knee?