Is this rash red? Did you have unprotected sex?
If not, if this "rash" is skin coloured or just slightly redder or pinker than your penis skin, it is probably normal.
Sometimes it is from the oil glands, or hair follicles, but mainly it's because all men have lumps, bumps, ridges, dots, etc on their penis.
If they become itchy, infected, get bigger, change color, etc then it may be time to see a doctor.