One possibility, even though he might not have asked, he might be one of the ones who wants a "look" and got caught by his girlfriend looking it up on the internet
Another possibility, he complimented your looks to his girlfriend, she didn't like it.
And another one, she sees you as a woman and some women don't like their men working with women.
And a fourth possibility, she is trans-phobic and thinks you will contaminate him somehow by being around him