Could be a phantom pregnancy? It happened with my lease mare. One day I went down to the pasture to find grey hairs all over her back and rump, she was in with a grey gelding. Then when the lease was up and I gave the mare back, I got a phone call a few months later asking if there had been any stallions grazing with her. I said no, but remembered seeing the grey hair on her back and wondering if she'd been bred. Apparently my mare was showing all signs of pregnancy, including an expanding belly, but there was no foal when she was ultrasounded.
This situation just brought it to mind. I'd definitely get her ultrasounded if you can, because that's the only way you'll know.
"When the power of love overcomes the love of power, the world will finally know peace." - Jimi Hendrix