I had this happen to me last year. I purchased a mare that the seller claimed was ultrasounded in foal. Well, no foal. I looked her up on AMHA and found 2 foals registered out of her 10yrs ago. I called the previous owner on the coggins and was told this mare had "changed hands a lot". I got really upset. I purchased her from a very well known reputible breeder. I had the vet come out and do an ultrasound and exam of her reproductive tract to find out where things stood. He said her reproductive tract was 100% sound. Based on the condition of her uterus, the size, no scarring, no nothing he figured she'd had no more than two foals in her life. He saw no reason that she could not concieve and carry a healthy foal to term.
She didn't come into a heat until early June. She was settled right then and there. I've had her ultrasounded twice and all looks well so far.
I would recommend having your mare checked out. Then you know for sure what you're starting with.
Good luck,
Candice