I'm in the 'trust your gut' camp.
We can pick up on so many little things that may have changed - everything including expressions, tone of voice, topics of conversation, opinions and attitudes, habits, timetables, personal grooming, weight loss and fitness, wardrobe, mentionitis, attention, kindness, cagieness ... When things aren't as they were - we notice.
Something is different, off.
So think it through.
Sadly, peri menopause can drive you nuts. I feel sorry for men sometimes having to cope with us trying to cope with it!
Is there a reason for cagieness - a big birthday, anniversary, Christmas surprise being planned? Are others acting differently too - mum, kids etc?
The relationship aspect - do some experiments and set things up. See if he's open and interested in advances and romantic moods. Might it be that you've rather lost interest and involvement yourself?
And investigate and observe. Look at the financials, the credit card and bank statements, the car mileage, unexplained absences, social media behaviour - anything off there?
But there may be no escaping the fact that if the odd behaviour is around phone use, you may need to do the thing that often is condemned here - yet can provide conclusive proof - and check out his phone.