What could cause anger and irritability?

Hello doctor,

I have a question regarding my friend. She is a very difficult person to understand. Most of the time she gets annoyed with basically anything and goes through moods. For example, if her bedroom door was locked and getting in her room when I was at her house, she would get extremely mad for not asking her. This simply goes with just about anything I ask her. She gets so mad and annoyed with me asking anything. She says my question is dumb or irrelevant. Sometimes too if I share a story I am proud of or just want to tell, she will respond with "Do I care?" And is trying to teach me not to tell her things at all. I want to share because it is human nature and she gets mad when I do. She also gets irritated with anything I do, and seconds later she is laughing and we are fine again. I am wondering if she is bipolar. She always is in a mood where she is annoyed, mad, and irritated and then five minutes later she is happy again and laughing. She would never admit that she has a problem, but I see it firsthand and know she does. Please suggest.