Unpleasant?

Today, I woke up and decided to feel better. It was working really well, then a customer told me that i was unpleasant after I told her to have a nice day. Now, I'll be the first to admit to the fact that customer service is not my strong suit and I've had bad days in the past but, I don't think today was one of them. I literally said "Large coffee? $1.95.....thanks have a good day!" apparently my entire demeanor is arrogant. I apologized and went into the back room to adjust to the shock. That customer actually hurt my feelings. The only thing that I can say is that I don't mean to be arrogant. This whole thing kind of brought me down but also made me think that I'm doing the best I can with what I've got. That counts for something, I think.