Slightly off subject, but I had a false pregnancy once, before either of my kids were born. Everything that my body did was screaming pregnant, but nothing was there. The doctor confirmed it. His words were "Honey, you ain't pregnant and you ain't never been pregnant". Eleven years later, and I still remember those words like they were spoken to me yesterday. I guess it's not super common in humans, either, because all of the social workers didn't believe me.
Anyhow, if this is a false pregnancy, when it's all over with, please treat your mare like she'd just lost a foal. I've been there and it was VERY hard on my husband and I, and I know that horses have feelings as well.