Oh, that feeling! My ex (separated 9 years ago, divorced in January this year, I found out last week!) was being so - nasty, dismissive and downright rude to me - which (23 years married) I put down to work pressures, he'd been told he was about to be made redundant. Some of the things he said to me were, looking back now, just appalling. Went on for 3 months. One particular awful Sunday after he went to bed I figured out the passcode to his online access and saw his Facebook messages with the wife of one of his old work friends - blonde, 33, two young daughters; He was also 49 (talk about mid-life crisis). Three weeks later I saw his phone bills (I'm more IT savvy than he thought) and he was still messaging and talking to her, so it was done, over, bags out the door. The 33 year old was horrified, she was only up for a bit of flirting (her marriage was done a few years later). He'd been going to bed early to chat with her, even at my works Christmas party, leaving early as he was "tired". Long times in the garage "tidying up". Walking the dogs poor feet off. Pffff!
As others have said, it might be something else but I'd trust your gut feeling. I was (thought we both were) in love and that pain - I wouldn't wish it on anyone. Well, him - and that 33 year old.