Hotels Last Minute Deals

My girlfriend planned a trip to Las Vegas and at the last minute I was unable to go. She didn’t want to reschedule, nor go alone, so she asked an ex-boyfriend to go with her. She has promised me a multitude of times that absolutely nothing will happen while they are together. They will be gambling and eating together, but nothing else.
I don’t believe a word of it. They are sharing a hotel room for the week and I don’t see how nothing will happen when I know that he still likes her and I once caught her going to see him when she said she wasn’t.
She tells me that when he goes to the room to sleep she will make sure she’s out and will only go to bed onc he’s gotten up and gone back out.
Am I wrong for thinking what I think and in no way trusting her in this situation? How would you feel or deal with it?