Oh dear. I hope this is not the case. Just know that horses don't 're-absorb'. If they were pregnant at all, they actually lose the fetus as they would in an abortion, although it would likely be too small to notice.
Congrats on your upcoming foal. Look into things to have on hand when she foals, Banamine is good for pain after foaling, she should also get a dose of ivermectin after foaling. And betadine or similar for the foals cord. We always increase mares feed in the last trimester of pregnancy. Sounds as though your vet has things squared away with her Rhino shots. Prayers for a safe foaling for you. I just saw where you were unsure above of her pregnancy hoping she is in foal for you