Mother's Day and my Single Mother Girlfriend

[Improvised]

I made the mistake (it would seem) of failing to get my single mother girlfriend a mother's day gift this year, and she is very upset about it. Last year I got her some flowers for mother's day and, though she thanked me, she acted surprised, as if I acted out of protocol--she even said something about the fact that she was not the mother of my child. Anyway, this gave me the impression that I was giving an unnecessary gift. I'm certainly no pro when it comes to gift buying conduct. Well, this year comes around and she's telling me how upset she is because I didn't get her anything for mother's day. She hinted at the fact last night, and I explained everything as I wrote it above--she agreed with me. Though, today she writes me an e-mail explaining how upset she is and that my logically explaining away the situation really hurt her (I had no idea). Am I to blame and should I try to make it up to her, or do I stand my ground on this one?