I feel guilty for getting the lapdance - could this guilt be the cause of this obsession which I'm pretty sure has caused this false memory?
This seems like textbook OCD to me, my friend. Best advice, try to stop ruminating over and over on the events as it just feeds into the OCD. As time passes, the worry will decrease and decrease. Hang in there.
It's amazing to me how much guilt can factor in. Truly. Yes, that can increase your anxiety. I see it around the forums all the time. Since you mention HIV, I can tell you that your worst case scenario of what you 'think' or more accurately "imagined" may have happened would not give you HIV. Oral sex is not going to lead to getting HIV. Giving you oral also imposes a low risk for other std's. That's all if that even ever happened which I'm really guessing didn't. You remember other details fairly clearly so you'd remember that.
What do you do for treatment of OCD? That's important. You can work with a doctor and therapist for a good outcome of that so these types of situations don't plague you.