I seriously doubt it is your letdown. The bottle comes out faster than your breast for sure. Since you are introducing the bottle, I would bet she has developed a preference for the bottle. My oldest did. It is more work to feed from the breast and the bottle just comes out so easily and quickly that they often just prefer it.
Many children get fussy at the breast when mommy is eating something that irritates them. It could be that you have introduced something to you diet that isn't agreeing with her. And now that you are pumping she may not be getting enough from your breast and thus is very frustrated. They like to latch on and have a full breast to tug from. Mine gets irritated when he's been cluster feeding and my breast isn't nice and full. He will pull and tug and growl and then eventually let go and fuss.
Another problem might be gas. Is she burping well after she feeds from each breast? Mine has become gassy after two weeks of eating fine. I have to give him Mylecon drops before he nurses to help with gas. I also think that I ate a type of taco spread with some little peppers and that it has irritated him.
One more thing you might consider. Are you supplementing at all with formula? Sometimes the formula gives them gas and that could be making her fussy everytime she eats.
If you are going to go to a bottle when you return to work, then maybe you should just pump and feed her that way if it makes her happier.