Is it normal to somthimes not like ure gF?

i've been 2 years with her now but somthimes like once a month or even just for a bit of time, i ask myself do i really like her. i really need some good help with this dilema it seems to prop up, at first i really like her and then i reconsider it, is there somthing wrong its like my relationship is at one point gr8 and then at other times normal and dull feeling (somthimes)