Brusing Underneath foreskin of Penis??

Well, erm....okay as the title of the question implies which, i cant hold back on being slightly emberassed about.

So i'll explain the best way i can.  I think it was about a day ago i did notice as i pulled down the foreskin over the head of my penis, what appears to be some mild discolourment (it looks a lil pale) and what looks like a bruise, its situated right at the bottom of the head where the foreskin ends when pulled all the way down, it is quite sensitive to touch and will sting if irritated, if i squeeze on my penis it aches..dosnt hurt that bad but, still, any aches n pains down there so to speak are something you shouldnt just ignore. now, i'm 23 going on 24 and i'm not ashamed to admit that i do masturbate, at least twice or 3 times a night/day, i've done that most of my teens and twenties and not really had that much irritation up until now, so.....what i'm trying to decyhper now is, that have i masturbated too hard?, or, did my partner while doing it to because it was pulled too hard?, it sounds like silly questions but, something has to have caused the pain, so far i've never had any STD's, i've always been clean.

i've seen alot of m,en report this problem online, so, does anyone have any thoughts or advice?, also i'm booked in to see my doctor tommorow to get it looked at.

XJ