I need to explain all this in depth so that you can understand and answer this question.
I have two best friends they are really important to me. We hang out so much and we do everything together. We all live in same apartment complex. I live with one of my bfs and the other one lives upstairs from us. I had this ex and we all went to hs together so he knows them very well. I am starting to think that my friend that lives upstairs from me had sex with him.
My reasons
1.) The other day my ex was at m house and we had a long talk and we put evrythng out on the line and told each other how we felt and during this talk he says and I quote... "There was one thing i did tho. I can't tell you what it is. This is soo big this would really stir things up and devestate lives and change everything forever."
SO THEN I STARTED TO THINK
2.) There was this one time when he spent the night upstairs... and I thought it was kind of weird that he decided to go up vs. down. I mean he was upstairs and he brought a friend with him. So I brushed that one off, but then he spent the night again a couple weeks later.
SO THEN I STARTED TO NOTICE HOW WHEN I'M AROUND THEY Don't REALLY TALK TO Each other. AND HOW WHEN HE IS CONCERNED ABOUT SOMETHING THAT CONCERNS ME HE Doesn't ASK HER HE ASKS MY OTHER FRIEND (THE ONE THAT LIVES WITH ME) OVER THE ONE WHO LIVES UPSTAIRS. THERE ARE SEVERAL OTHER THINGS THAT MAKE ME SUSPICIOUS, BUT I Don't KNOW IF I AM OVER ANALYZING THE SITUATION OR NOT. OH AND I TOLD BOTH OF MY FRIENDS WHAT HE SAID AND THE ONE THAT LIVES WITH ME Didn't OVER REACT AT ALL SHE WAS JUST LIKE I Didn't DO ANYTHING WITH HIIM. THE OTHER ONE Kind of BRUSHED IT OFF. I Don't KNOW WHAT TO DO. OR HOW TO GO ABOUT ASKING THEM. OR IF I SHOULD ASK AT ALL. I Don't WANT TO OFFEND MY FRIEND BECAUSE IF SHE Didn't DO ANYTHING SHE WOULD BE UPSET THAT I WLD EVEN THINK SHE WOULD DO THAT. AND That's THE PROBLEM IM HAVING BECAUSE HOW CLD I EVEN THINK THAT MY BEST FRIEND WOULD DO THAT? AND MY OTHR BF EVEN THINKS SOMETHING HAPPENED! WHAT SHOULD I DO?

Can someone help please!

I think the best thing you can do is talk to your friend. Stay calm and level headed and choose your words well. Don't jump into a conversation with accusations. Point out that you noticed things seems a little odd between them, and ask if anything happened that made it that way. Perhaps they got into an argument and that's why they don't talk. There could be other reasons for the behavior, but when you have a suspicion, your mind tends to create and manipulate minor things that support the suspicion.

Did I read that right, your EX? He's your ex. You have no say in who he sleeps with. Just because things didn't work out between you and him doesn't mean him and you're friend shouldn't be allowed to try. Buuut, if they are hooking up you're friend should be able to talk about it with you. How would you react if she were sleeping with him? Maybe that's the problem?

You're going to hurt people, he's your ex. Leave it where it is. Things like this have a sneaky habit of coming out. You'll find uot if something happened.

I have been friends with my friends boyfriends in the past and asked them not to tell me about their relationship because it made me uncomfortable trying to give advice knowing that it was with for/against a friend, it may be this?

It may be a multitude of things, if you feel like you can't deal with this yourself then ask, but you will be causing huge problems for yourself and it may be nothing.