Here's what you do if it happens again. Get out of the bed, or off the couch or out of the submarine, wherever you happen to be at that point. Walk over to the nearest corner, erection in hand and just start jerking off whilst staring at her like an insane mass murderer type person.
If you could cry as well, that would be - like - awesome! Not just a few pussy tears running down your cheeks that she won't even notice. Be a man about it an cry like you're 5 years old and your mother wont buy you the batmobile. Resisting the urge to get on the floor and start kicking and screaming of course.
When and if you cum, it'll probably be quite the challenge. Make it sound like you're in tremendous pain - maybe let out a few stray farts just for good measure, they'll just add to the overall aura of your performance. Whilst of course, still crying like a weak little girly man. Then, once it's over whisper softly, but loud enough for her to hear "you did this to me".
Then get back into bed.