Penis head sticks to underwear, reddish spot in the opening

Hi!
Around 2 months ago, I noticed a pink-red color in the opening of my penis, but only in the lower part of the opening (the upper part of the opening is ok). It was weird but it didn't bother me.
But around a week later, the opening of the head of my penis started to stick to my underwear. It happened randomly during the day, and I have to pull it away from the underwear, and it hurts, kind of like stinging.
It's not because of discharge, it feels like the underwear get somehow stuck in the opening.
The lower part of the opening stings in certain places when I touch it. Sometimes it looks a bit 'larger' (swollen?) and the red color is visible, and sometimes it looks normal, and to notice the red color I need to spread the opening.

It doesn't hurt when I pee\ejaculate - except for one day when it was because I probably made some damage when I pulled the head from the underwear. But it passed the day after.

The penis looks ok, nothing different from before. I should mention that my skin there is pretty dry. Also, I am circumsized (from birth).

I post now because when this happened today, I made a (tiny) wound and it starts to scare me.

I know I should probably see an urologist, but I read about similar problems that urologists weren't able to diagnose and it worth a shot to try here.

I'm 18, haven't had sex so it rules out STD's. I take roaccutane 4 months already.

Hope you'll be able to help.

tl;dr: penis sticks to underwear, it stings and hurts. a bit red in the lower part of the opening, and it stings there. circumsized and virgin.