Why Could I Have Bled From Penis During Intercourse?

hey, I'm male, 21, weight 75 Kilos and have no relevant medical history. My problem is the following: During sexual intercourse my gf's skin got in touch with my sperm for about half an hour. It was only then that we realized that red stains had evelopped right where my sperm had been. First we thought about an allergy, but now i have found out that I get these stains too when my skin gets in contact with my sperm for a longer time, what can it be?