I dont know whats wrong with me..? my weight changes everytime i eat something small ?

this morning i weigh myself to see that i was 150. then i ate one banana with half a cup of water... to my curiousity i check the scale and i saw that i gained 2 pounds.... everytime i eat something fairly small... why do i gain so much? and how can i pervent it? im on a diet. thanks =]