Why do guys always do this to me ?

Well usually if I talk to a guy or meet a guy, they are always shy and nice around me. Theyre polite and always stare at me and give excuses to talk to me. Then 2 days later they turn into a**holes and make me do everything for them. Then they start calling me b**** and sl** and w****. Ughhh its really bugging me! Why do they always act like that and make me seem like im their slave or something?