I found a post by my so and now I feel terrible
What was the post about?
There's a picture of my ex on his facebook
I completely get why that would be upsetting, but chances are it's nothing to be worried about. You should talk to your partner and let them know that the photo you saw doesn't sit well with you.
He was actig suspicious and I wanted to catch him.
One of the main things when in a relationship is trust, and when that's violated, it's hard to come back from. Rather than trying to catch him, I'd suggest you talk to him. If there's a valid reason for him being caught, then perhaps ending the relationship might be a good thing to do.