Please see a dermatologist. It's not uncommon for men to get issues like Balanitis, or the veins to become more visible after sex, or become red for a while. It is most likely nothing serious, but please see a good dermatologist, He can put your worries to bed. Why keep on worrying for a long time, when a simple visit can solve it ? Think about it.
Look up Balanitis. Could be what it is, but I'm not sure.