Oh sweetheart, I'm so sorry....
I've been thinking... I know Potomac was ruled out, but could it possibly be another form of Colitis? Depending on how long ago you bought her, and how far away her old barn was, do you think it could be a big enough change in plants and roughage in the pasture to change the bacteria in her intestine? If this is the case, I know theres no magical cure. But, one [possibly] helpful solution I was told today at the barn is to mix the manure of a healthy horse, into a bucket and water, and give it to her through a nose tube. Considering the healthy horse obviously has the correct bacteria in its digestive tract this could help to bring the bacterial balance back to normal. I've also heard of the use of yogurt, although I haven't heard anything serious about that, just shadowy mentions.
Also, I found this. Merck Veterinary Manual
its a link to the Merck Vet Manual, has quite a bit on intestinal issues. It may help.
I really hope she makes it through the night. My hopes and prayers are with you, you're mare, and the unborn baby.