Hmm. Actually, I think your question is quite correct. Women hate being called a bitch or bitchy because it just sounds mean. But, you are asking why we nag, or get upset or angry easily, and it's perfectly ok to ask that :)
I think it's because *most* women want you to know how their feeling and to do something about it. In most experiences, we've learned that just saying "Hey, this is how I feel" just doesn't get the job done, so we *act* on it instead and emphasize it to make you notice more.
We also get pregnant (which will be the worse time to see a woman bitchy...just as my husband haha) and we have that time of the month. During both those periods, our horomones are going crazy and we don't always know why we act the way we do either. Right now, there are times I want to bash my head into a wall I'm so irritated, and I'll be at home alone just doing housework or something and I really have nothing to be irritated about. But my hormones are going INSANE right now. Is that an excuse? No, not really as I always control myself when I did have my time of the month. It's just A LOT more now and it's something different for me. I do cry more than anything though lol :)