What could be wrong with my mom (bipolar)?

I'm afraid my mom may have bipolar, or some other mood disorder. She will be super happy, on top of the world one minute, and then the next she'll freak out over something little. For example, yesterday I brought up that I wished she had gone with me to go buy something (as part of my summer goals list), and she freaked out, calling me an ungrateful little b**** and saying I was going to be a failure in life for depending on other people (keep in mind, I'm 15, so I couldn't drive myself, and I have a 4.0 GPA). Since yesterday morning when she blew up, she hasn't said a word to me or my dad or my little brother, who's 8. Even when he (my little brother) says he needs help or something, she'll just sit there and wait for me to do it. All she's done is sit on the bed in our hotel room and read her book. She won't eat, even though she made a fuss about going to a sit-down restaurant (before blowing up). And this isn't the first time this has happened either. She drinks a lot (for no reason, other than she feels like it), and gets all defensive when we tell her she needs to stop after the 5th beer. In fact, she's extremely defensive in general. When she's not mad or drinking, she's super happy, and offers to make everything better by taking me shopping. And even when she is mad, she'll purposely treat other people (like when she has an episode in a store) super nice and friendly, and when we get back to the car she'll keep yelling. And once she's started yelling, she won't stop. She'll keep repeating the same things over and over, and bring up things that happened, like, months ago. Then a day or two later, she'll act like it never happened and be super nice again. It's gotten to the point where I don't like leaving her home alone with my little brother, because she's threatened to just run off and take him with her. I know I'm kind of rambling here, but I want to be thorough. I'm thinking it may be bipolar because her side of the family has a history of it, including her aunt and her grandma. I don't want to bring up going to a psychiatrist, because I know it will tick her off again. What do you think? What can I do to make it stop?