I had a painless red bump on the shaft of my penis and i am worried on whether this could be a syphilis chancre. had unprotected oral about a month ago. how does a typical chancre look like? and if the bump is painless, does it always mean syphilis?

Syphilis chancre: Chancres are open sores, not skin bumps. However, if it's been under a week or so, maybe the bump will begin to ulcerate, becoming a chancre. With unprotected oral exposure a month ago, the timing is right for syphilis: chancres generally begin 2-4 weeks after exposure. Does a painless bump "always mean syphilis"? No, usually not. However, you definitely should be examined and tested ASAP.