What Causes Erection Problem And Pain In Penis?

HI i was having sex with my girlfriend about 3 weeks ago and she sat on my penis while she was on top of me and i was fully erect! I went instantly soft and was in pain but i doubt there s any tissue damage or whatever because its wasn t bruised or anything but i feel the time from time to time. Its been hard trying to get an erection we haven t had sex in 2 weeks and I m wondering how long it will last ??