Red marks on the head of my penis.

I have red marks on the head of my penis, they do not hurt, burn or itch. There are no symptoms of anything else like burning when I urinate. After me and my girlfriend fool around they become brighter as if the blood stays there after its over. I have recently tried putting off any exacerbation of the problem with intercourse, however recently we started again. It was very faint before intercourse, now it is just as vibrant as before.