I started feeling uncomfortable in the men's room well before I had bewbs...once I started wearing makeup and "showing" more, both I and the men were clearly uncomfortable (this was at work, with familiar faces). I waited until I could change my gender marker on my d/l before I started using the women's room.
I read on here somewhere, "just walk in like you OWN the place", and it's worked for me every time. Once as I was leaving, an older woman remarked "this is the ladies room", and I replied, "Yes. Yes, it is."