I'm guessing she first thought she was simply doing you a little favor.
She sees a guy buying panties and she thinks maybe he feels a bit embarrassed buying panties for his wife/girlfriend, so she decides to make a little joke.
She wanted you to know that she didn't think you were buying them for yourself.
And then when you said "Actually they are" for you, she may have thought that she made things worse by "forcing" you to reveal your secret.
That's why she felt so awkward.
I think she meant well, but as we all know, the road to hell is paved with good intentions.