When my girls are pregnant, they get very picky, almost to the point of not eating much at all at about 3 weeks post breeding until about the 5th week of their pregnancy when they will finally start eating MUCH better!! Then at about 7 1/2 - 8 weeks they get a little picky again. I don't think it's picky as much as it is just the fact that their belly is so full of growing puppies, there is not as much room for food. That is when I will start feeding them less at each meal.............and add an extra feeding or two throughout the day.
Congrats on your upcoming litter & good luck to you!