One minute my girlfriend loves me and we are at our best joking and laughing and everything is ok the next she?

acts as though she hates me..she says hurtful things like im dumb for being with her and she doesnt know why she has been with me for as long as she has and she is tired of me and she will not change her ways for me and calls me idiot, dumb b*tch, ugly, then a day later she loves me and we go about our day like it never happened and then its lke eggshells i say somthing she doesnt like and it back to the hurtful things being said all over again...why is she like this with me??