Probably because you said something rude or used foul language. Just a wild guess.
Her body is going through shoot of changes. Changes that she doesn't even fully understand. She has allot of additional hormones going through her body as well as the stress about having a child and growing it so it will be happy and has the best shot at life. Her emotions are totally out of wack and she honestly can't help it. Try to understand that it might not be you she is snapping at our getting upset with it might just be her body responding to a situation. I emirate toy to get a pregnancy book for dad so you can understand a little more what is happening. My husband has been wonderful and so understanding despite my moments of crazy (that happen far too often). She honestly can't help it. Just be supportive. She is carrying your child, of all the times to let things slide and roll off your back, now is it. Good luck