I agree that the gut never lies, but the brain hears the gut and can really run with it! So if your gut says something is off then something probably is, but it may be as simple as a weird disconnect period or there maybe some hidden guilt about some wrong doing but it could be something small and forgivable or the worst possible scenerio or anything in between. ..I guess what I'm babbling on about is have you tried talking to your partner? Have yu told them how you feel? If yes, what about their response are you still questioning? If not, are there prior indiscretions in this relationship? And what other information, vibes, clues are you recieving outside of your gut that is reinforcing your suspicions? Check all your boxes twice and if it still seems fishy then walk way.