Dryness? If this has happened since before you ever had sex then it's how your body it. If it started after sex, it might be an infection of some kind.

It sounds like it's just dry. Put lotion on it every single day.

it doesnt match any symptoms of any sti's

and it's only happened within the last year, it occasionally appears on my testicles too, infact thats where it started.

could it be a reaction to washing detergent, saliva, or even condoms?

it is really annoying

Psoriasis maybe? Yes, it could be a reaction to any and all of the things you listed.