unfortunately your description of a "red painless flat blotch" on your penis is not sufficient to make a definitive diagnosis let alone offer treatment suggestions. this could be balanitis (you can look this up) but this is just a blind guess since there is no way to examine you over the internet. if this condition does not self-resolve in a few days you will need to seek a hands-on medical examination.
two red painless and unraised blotches on my penis tip? . yesterday i masturbated cleaned myself clean with a wet wipe. an hour later i went to the toilet and saw a red blotch on the tip of penis just to the right. it is a brightish red about the size of a small pea it is not painful or itchy. this morning i inspected more closely and found another much much smaller one (although its possible i missed it before). it is not as std as i have not had sex in over a year however i am eager to resolve this problem as i am dating and it isn't very pretty.