When i pullback the forescin of penis, the skin on inside is red on one side and a little swollen , look like a rash. came onset, 18, what could b?

Yeast: This is a yeast infection. It should be easily treated with anti-fungal medication.

Possible yeast: Yeast infections can occur under the foreskin and produce redness of the penis. Have you had any exposure to a female partner who may have had a vaginal discharge? Either way, keep the area clean and dry, and apply an antiyeast cream like Mycostatin (nystatin) to see if the redness clears. See your physician if symptoms persist or worsen.