Actually, same thing happened to me. Once, I was buying some boxers, and the cashier said "have a nice day, ma'am," so pointedly I had a feeling she was trying to make some sort of comment about my masculine appearance and purchase. And I know I wasn't just imagining it, because my boyfriend was with me and agreed she said it in a fairly nasty manner.
I've gotten the "poor little butch" reassurances too, but they didn't bug me nearly as much as whenever people felt the need to practically shout a title or pronoun at me, as if to remind me of my gender.