Search Results: get back at ex after breakup

So i got dumped a couple of months ago by my long-term girlfriend (we were together 4 years). I do not have concrete proof but I have doubts she cheated on me with my best friend and left me for him, but they are hiding it from me. she also talked a lot of trash about me after the breakup, even stupid posts on social media obviously intended for me.

after our breakup my friend went abroad for a couple of months and right before he came back he dumped his girlfriend all of a sudden even though just a month ago he was so in love with her. ever since i have noticed that he and my ex have been quite in sync on the social media, coming online and offline often at the same time. not all the time as they probably converse with other people also but too often to just be a coincidence. or he will just turn off his active status right when she comes online and turn it back on after she goes offline. i didn't look at it before but since i noticed it now it's hard not to honestly.

we hung out a couple of times since he returned and something is just off, i have a bad feeling and my instinct is telling me he is hiding something. always keeping his phone really close to him, dodges the convo when i start talking about my ex. i once even got into a bro code talk due to something else entirely but it came really good as i could see his reactions and he just wouldn't even say anything kept really quiet and kept glancing all over the place. in the last couple of weeks we didn't see each other but he reached out to me twice by text but all of those seem forced and short and really like he doesn't wanna do it but has to. i don't actually want it either, would prefer he didn't.

As i do not have any evidence and all i have is doubts how do i approach this. i don't think i would get anything out of asking either one, pretty sure they would make it look like i was crazy. If i am right i obviously don't wanna have anything to do with either one of them.

What should I do?