I can't give odds here with all the variables, and I don't know what the thread you read was, and what that scenario was.
I would say that I would be highly surprised if this led to an infection.
No, you might not see a lesion in a couple of days if you ever got infected. It can take up to 20 days for symptoms to show up, if they do. Don't let this freak you out though - the chances of you getting anything from this are very, very low. I don't really know any other way to say you don't need to worry, and your chances are very low.
So in theory, this could transmit herpes - the thong could slip, etc. Your skin on your butt is thick enough to not cause transmission in most cases, and you don't know that she has herpes.
It's not something I would worry about at all, and wouldn't test. You are unsure if her vagina came into contact with your anus, which is something you'd feel, so try not to create a bunch of "what if" scenarios in your head.