It is possible to have both, and it is also possible for one to look a lot like the other sometimes. Anxiety goes along with a lot of mood disorders. One difference is that GAD tends to be most of the time, while bipolar has distinct episodes of depression and mania and during those episodes people can notice that you are not like you usually are. It is ok to ask the person treating you why they don't think you have bipolar disorder. They should be able to explain it and if they missed something you will have the chance to explain what they missed. Either that, or you will have a better understanding of your diagnosis. It's always good to ask questions if you aren't sure about something.