Well, typically mares would not be coming into heats this late in the year, so it wouldn't be surprising to not see her in heat. In fact, my mare has never displayed obvious heat symptoms, not in spring or summer - some just don't.
If she is bred, the foal would be several months developed by now - so would def need to confer with your vet as to if it is even safe to abort... either way, the vet would be your only guaranteed way to confirm a pregnancy, so I would start there.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela