It is a possibility. I hope not for her sake but if she is, you will just kinda have to roll with the punches. One thing that I have noticed watching all those animal rescue shows is that usually, a pregnant mare will look worse with the same care as other horses because she is giving more of her nutrients and such to the foal than she is taking for herself. Many mares who are skinny/malnourished will have a healthy foal. Often, the foal runs into problems when it comes time to eat because the mare doesn't have enough/proper feed to produce good milk. Those are usually still in the bad environment though. Even if she is preggo, I am sure that she will be fine and produce a healthy beautiful foal.
Especially since she is now with someone who will actually care for her.