I have asked for advice regarding jealousy before. But now I'm concerned that I'm "seeing and hearing" things.

I will explain:
Last weekend I went out with my boyfriend and two of my female friends. I made the mistake of initiating a threesome with one of these friends and my boyfriend about 5 months ago after a night of partying in which I was already getting jealous of how much her and my boyfriend were talking and looking at eachother.

Now whenever we are all together, I swear I catch him staring at her all the time. I could have sworn he was looking at her in the rearview mirror when he was driving and then would quickly look away if I saw it. Also it seems like whenever she's walking towards/away from us he stares at her. It seems like they have more interesting conversations, etc.

Anyways on the weekend we were up drinking together and I was pretty drunk and my boyfriend and both of my friends were saying I should go to bed. I hate saying this but I don't trust anyone. I can't help but think they were trying to get me to go to bed so they could all hook up. I do not want to think like this! So anyways my boyfriend came to bed with me because I'm sure he knew I'd get jealous if he stayed up with them. I was laying there and I swear it sounded like he was trying to inch away from me slowly so that he could get back up and go out with them. I made it seem like I was sleeping to see what he would do. He didn't end up getting up but it seemed like he was so close. And his phone went off a couple times and I thought it would be them so I "woke up" and freaked out on him and asked to see who texted him. It was some ad thing from a messenger. Or so I think.. maybe he has an app that hides his texts..

Anyway when I told him that I thought he was trying to get up he said that I'm psycho and imagining ****. But the way he said it seemed like he was lying. I seriously thought that he would sneak away and I would hear them all doing sexual things. Why do I think like this? Why do I "hear and see things" that aren't happening? Does anyone else have paranoid delusions involving their boyfriend/girlfriend and how on earth do you cope with it when you seriously hear and see things that make you think something is going on..?