My Excuse For Being Rude

Ok, so we had lunch with Dr. McCloskey with the econ club Thursday. She was a great speaker with some very interesting points about economics and economists. This isn't why I'm writing, however. I'm writing because I have found a serious flaw in my personality.

I get to the table with my food and she has already sat down. I realize that I don't have a soda, so I put my food down, talk a second, then excuse myself to get one. On the way there, I realize that she didn't have a soda either. So now I have a problem. I SHOULD have asked if she would like one. This makes me look like a thoughtful and polite person. But I didn't.
Why didn't I?

I assumed that if she wanted a soda, she would have gotten one for herself.

They were free afterall. It didn't occur to me until I was pouring mine that the machine was kind of out of the way and I had been here before, so I knew where to get the drinks. Anyway, I felt stupid, and I couldn't very well go ahead and get her one, because I didn't know what she wanted. So... I suck.