What Causes Pain In The Penis After Intercourse?

I am suffering from a typical problem...after intercourse (i am not married) i feel pain in my penis though not severe, a burning sensation inside the shaft of the penis. It becomes loose and oversize, specially after masturbation. It took 3-4days to become normal again. Previously I was used to masturbate 3/4times a day. Can you suggest how to get rid of this problem.