Search Results: chatting online

So I've met this guy 4-5 months ago and since begining acts different than other guys in our group of friends. He had problem talking to me, he was staring at me a lot and smiling, when I would ignore him standing next to me he would become mean and stuff.. and always was near me wherever i stand. Last 3-4 weeks he's been really distant in person.. he would smile anymore, won't stand near me and other stuff he did, but he always wants to help me. We have group chat with two more guys, and my guy friend said that him and I text almost always in there. We tease eah other, he compliments me and we sand each other hearts sometimes (most are between us).

So few days ago he called me on app and me and my friend didn't answer and he said that we should give him our numbers and i sand it to him in private box. After two hours he texted me and I tried to continue conversation but it died quickly (I have his number for month and he has mine but he acted like he forgot about it and he sent me text). He stopped to talked on wapp and answer to my text on group chat from few hours ago and i ignored it. I ignored him for 3 days and today I tagged his friend and asked him something and this guy answered in second for friend, and he did it twice.. and he wasn't even online when i sent message in group chat.

I don't know what's whit this guy. I think he might be like me but im not sure. Two weeks ago we were for 5 minutes alone first time and he was so quiet until his friend came and i don't know why.
He's nice guy, but I don't know why he won't talk to me in private box or when we are alone. He's really sometimes weird. What should I do?

Long story
I was in a relationship with a married man (he has been with the wife since 2014) for 1 year.
- had access to his email for some reason
- saw that someone sent him nude photos - he brushed it off as spam
- dug a little and found out he was on dating sites, and also posted ads looking for sex in the past
- when questioned, he say he did not meet anyone in real life. And that all he did was chat online. Fine. Since it was way before we met.
- I found that he recently (Aug 2020) commented on a new sex ad. There was a email notification sent to him that someone replied his enquiry. He denied it saying that it was email spam

Is this what they call once a cheater always a cheater?
Should I let the wife know?