Back of knee pain

I ran nyc two weeks ago and ended up running a half marathon this weekend in philly. Around miles 11-13 my right knee started to hurt which was surprising because I didn’t have ANY knee injuries during my marathon training cycle or even after. The back of my knee I can barely bend without pain. Maybe it was too much in two weeks? Bakers cyst? Looking for any info on how to prevent from happening in the future!