What kind of knee injury is this?

Hi,

I hope someone can help. I've 2 nagging injuries this year, but the one in my knee is confusing. When I run with exertion, sometimes I will get a popping feeling in the medial posterior of my left knee. It occured after I bought some Newtons and stupidly ran a 7 mile run. Then my lower left calf started hurting and the back of my left knee started popping. There is no pain in the left knee, only this bothersome popping feeling. It seems to be going away, but surfaces every now and then. Any idea what it is?

Timmer