What should I do if I think my friend is cheating on her husband with a guy at work?

I have a friend I've known for two years that I also work with everyday. My friend is a married women with a now two kids under 2.

In the first year we got to know each other we developed intense feelings for one another and shared an intimate moment at work where no one knew anything or saw. She ended up finding out she was pregnant and she had no choice but to break my heart, but we continued to be friends. Mind you, I am a female.

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I've been having a hard time letting her go I still want to be friends. But she doesn't talk to me like she used to even around work, but I notice that she seems flirty around another male coworker and it upsets me very much. My intuition says she's messing around with him. She looks so interested in what he says and just looks flirty to me. I do have anxiety and depression so I don't know if my feelings are just insecurity or there is something really going on. I know some will say its not my business but I feel after what she did to me that it kinda is my business or at least I'm trying to be a friend or I'm just jealous of it all.

I don't know if I should tell her or keep it to myself. I think I still have feelings for her because this all makes me mad. She broke my heart because she had to think of her family but yet she looks like she's giving her attention to another man. Because of our history What do I do? Do I say something now? Or should I collect more evidence until its time?