I don’t think that anything here means that she’s cheating. It’s possible but not definitive and you need to do more investigating before you come to the conclusion that she’s cheating.

There is something she’s not telling you but it doesn’t mean she’s cheating. She could be pole dancing. She could be part of a secret cult. She could be secretly taking piano lessons to surprise you with a song she wrote for you. It could be anything.

There are plenty of things people do besides cheating that they’d never want their partner to know about.

You should set boundaries, for starts you should tell her that as a married couple with a joint bank account, she is not entitled to go MIA and spend money without explanation. She is of course entitled to independence but she is not entitled to harm you, your finances or your emotional comfort in the process.

Losing weight and gaining confidence does not make a person cheat, in fact it would make them more inclined not to. Majority of cheaters are insecure, and the rest of them are narcissistic. Cheaters typically are seeking either validation or they feel entitled to experienced that others aren’t.

You need to ask yourself this question: what can my wife do to help me believe she is not cheating? Think of those things and write them down. Review them and make sure they are fair. Then tell her you need her to do these things.

If you cannot think of anything she can do to change your suspicions, it is likely paranoia rather than cheating, because paranoia does not ease with reason, while suspicion typically does.