I don't know how long you have had her, but it's possible she ate something before you got her, maybe in the foster home. A friend of mine had her dog for a few weeks and threw up a Barbie shoe! She has no children. She found out her dog had a stop over at a foster home before coming to her adoption group.
My girl Boo threw up a tail of a stuffie toy that was the size of a golf ball after I adopted her. I asked the foster parent about it and she doesn't have toys at her house because her dogs destroy them. Boo must have gotten it before she came to our group.
But then again, she could've gotten something at the dog park.