Why blame your bitchiness on pregnancy?

I was at Starbucks today and the lady in front of me ordered a complicated drink. They made her drink wrong and she flipped out. She might have been 7 months pregnant. It wasn't a normal flip out she really insulted the poor barista. She offered to remake the drink and the lady said "if you can find some one who is competent at doing their job, I would like them to remake my drink" I was shocked and she continued she looked around and said to the people in the lobby including myself she said "was my drink that complicated?" I responded with no but ur attitude needs an adjustment. She said "well I'm pregnant and it makes me grouchy" I told her I was pregnant too but that still doesn't give me the right to talk to people the way you did. She was so upset and walked out crying. I just don't understand why blame it on the pregnancy.? You can still be a decent human being.