It is unlikely to be an STD if the condom remained intact the whole time, but any new symptoms or pain is worth getting checked out by a doctor. You could have something else going on totally unrelated to the encounter that needs medical attention.
Your penis may have been wet from your ejaculation or the lube on the condom. It's very hard for us to say what happened as we weren't there. Just go get things checked out, and let us know what happens. :)