I just don't know anymore :/

One minute I'm sure something is going on, the next I'm confident and relieved nothing is going on. One minute I think I'm right , the next I think I'm crazy. This morning, I notice that my husband does or uses a particular thing to improve his look. I don't want to explain it exactly, but I just noticed. He wasn't hiding it per say, but it tells me he is trying to impress someone. Who? Why? 
He sits around here all weekend.. no worries. Time to go back to work, and he's all about showering, and trimming, and cologne. I don't know if it's in my head.
After checking his phone, before this, I noticed his contact with his female co-worker seems to be all about work and mostly platonic. And he acts very attracted to me, and interested. Plus I would think he would be calling this person every chance he got, behind my back. Maybe it's just someone to flirt with at work. I don't think he is attracted to this girl, but maybe that is why he finds it "safe." Maybe because he introduced me to her, a long time ago, that he won't have to worry about it going further than he wants. My husband very much so likes flattery. He would probably would be just as happy with flattery, plus I'm sure he finds flirting is not doing anything wrong. I thought about writing him a little note warning him.. that if he cheats, it's over. Pretty much a one liner. I don't want to mention her name.. or make this about her, because really it's not. It could be anybody. It's about me and him. Plus if I'm wrong about her, then I don't want to put it in his mind. I think just a generic statement about infidelity would be better. I don't know, I just want this to be over. I want to be at peace, but I think this is about to get worse before it gets better. :(