Stereotyping or being racist?!

On Saturday I was at work taking peoples food orders ...The day was going normal until this old white lady showed up and I took her order but she kept changing what she wanted. So everytime I repeated what she wanted she would get angrier and angrier, I dont know why, b/c all I wanted to do is get her order right. As I was repeating her order she started yelling at me and asked one of my co-workers if I could speak any English or am I just dumb. At first I was speechless, I could not believe what I had just heard. All I could do was yell out "Yes, I do know English", after all I am at work. I dont know if she was impatient or it was because she saw that most of us working were hispanics and she just assumed we were those "wetbacks". I've had many racist comment thrown at me, but this one I'll never forget. Maybe b/c in other occasions I snapped back, but since I was at work I felt and was powerless.