Serious, personal family question. Advice/suggestions needed! :(?

Basically my step dad and I are really worried and concerned about my mum. She has become someone completely different to who she used to be. She goes from being in a normal mood, to a angry and horrible mood in the blink of an eye. As each day goes by, it becomes so much more frequent and apparent. When she goes into this angry mood she says horrible things that you wouldn't say to your children or partner. She also can become very aggressive. She also often says silly and stupid things like "no one appreciates me cooking dinner every day", when we clearly do. Another example, I could clean the whole house for her, i do the dishes every day, I play with my little sister and clean after her, but somehow in her eyes I'm the laziest person ever. I don't understand, I really am not lazy. She doesn't seem to appreciate anything. She also gets mad at the weirdest of things, and causes huge arguments out of them. She is the one that always has to be right, and everyone else is wrong. I understand that my mum is under a lot of stress but I know this isn't just stress, this is something else.. there is something wrong. I love my mum more than anything, but we used to be very close and we seem to just be becoming very distant. I am seventeen years old, I am not some young "kid" who thinks they are perfect and that their parents aren't.. The problem really is her. Its causing our family to break apart, our house can never stay happy for more than a couple of days because its always argument after argument because of her behavior. Does anyone know what could possibly be wrong with her? Help is greatly appreciated!