How often are you feeding this girl?
Yes, and what are you feeding her?
If there is a lot of bird food (especially grain and corn) on the ground and bird droppings I think it would be best to keep the dog away so she doesn't eat any of that. Otherwise I think it unlikely she has "caught" anything from the birds.
We feed birds here too and our Westie sometimes gets some pieces of fat from the suet that falls to the ground, and we do not feed the bird any grain. There is no indication he is getting sick for such feeding episodes.
Same question as Karla. Sounds like bile, try giving her a little food later in the evening or cut her night feeding in half, one feeding at the regular time and the other half a couple of hours later. See if that makes a difference in a couple of days.