feeding problems with 3 month old baby.?

My daughter turned 3 months last week.She is exclusively fed & eats every 3-4 hours. Since the last week itself, she has been eating less. She nurses from one & when I change sides, she refuses to nurse from the other. It is clear that she is hungry as she sucks hard on the pacifier or puts her fist in her mouth, but will not come back to my at all. Since yesterday, I have been pumping the side she doesn't nurse from & feed her the milk from the bottle. She gulps down the whole lot, as much as 100ml. I don't understand why she will not eat from my even though shes hungry. Has anyone had such an experience? Advice please.