You’re tying yourself in knots trying to prove you haven’t done anything wrong, when the real issue is that your mum is anxious and hasn’t articulated why.
Nothing you’ve described is alarming. New relationship, a planned day out, child-free, a few drinks, home safely. Given what you’ve said about her drinking history, her discomfort around alcohol generally, and the way she applies different standards to you and your brother, this looks very much like projection. Stop internalising her unease as evidence that you’ve messed up.
If she has a concrete concern, she needs to articulate it. Is she worried about your kids? About frequency? About alcohol because of her own past? Does she simply not like the fact that you’re happier and changing? “I’m worried about you” without context is just nonsense. Have a direct conversation and her what, specifically, she thinks is the problem, and what she is actually afraid will happen. If she can’t answer that, then just disregard.
You’re allowed to be happy, to change and to enjoy yourself without it being pathologised, especially when your children are cared for and not involved.