I would have a vet check her and if she is, have your vet instruct you on how to properly care for her. He may have you add or subtract things from her feed and do things a little differently to make sure she is at her optimum health.
Thanks, I don't know if she has had a foal before... I have only had her for 3+ years and she is my first horse so I have no idea. The people we bought her from had a cows and they had calves. She wouldn't leave one of the baby cows and was mothering one, but I don't know if that helps.
Agree with Bubba. You need to have the vet out...yesterday. If she is preggo, I would discuss having the pregnancy terminated. Asking a mare that old (that hasn't been kept bred in recent years like a broodmare) to carry to term is just asking for serious trouble. Just like women who get pregnant when they are older, the risk of complications to both mother and young goes up exponentially. I certainly wouldn't want to risk the health and life of a beloved old friend on a completely unwanted, and likely unmarketable, foal.
If it is too late to abort, then you need to have an in-depth talk with the vet about how to provide her with the best possible care to give her the best chance to have a safe pregnancy and birth.