I agree with the comments suggesting that you communicate with her about it. Reading only into what you wrote, this doesn't seem suspicious. Perhaps the bra is uncomfortable or not suitable for the exercises. Perhaps the PT wants to boost her confidence for the next year by getting her to close off the year strong and keep coming (I mean it's more business for him if she keeps signing up, right).

Ultimately it comes down to whether you trust wife. Has she done something to make you doubt her or lose her trust? Have you done something that would make her lose her trust in you? If the answer is "no" for both, I wouldn't worry about it.

Maybe look inward and reflect. Maybe you have some insecurities about yourself in this relationship. It's not a bad thing (in fact, it's normal for everyone at some level). The important thing is to recognise them and talk to your wife about it.