Have you gotten an STI/ STD even though you were using a condom

I enjoy casual sex and always use condoms, but I am a little worried about getting something by hooking up with random strangers. I am a college grad recently out of a very long term relationship and I just want to have some fun, but I want to be safe about it. What are the chances I will still get something even if he uses protection? Yes, I know I can just ask him if he is clean but I do not want to take a strangers word that he does not have anything.