We had a filly with similar symptoms, first obviously depressed and then by the time we loaded her in the trailer and got to the vets she was down and started seizuring shortly after. She had something called Tyzers, which is a bacteria that always lives in the environment, but occasionally a foal is susceptable to it. It destroys their liver. Another foal I saw with it was at the clinic with his mom for breeding, when the owner came to get him he was a little dopey, but since he was a very quiet baby nobody thought anything of it. By the time they got home (which for them is less than 10 minutes away) he was down and they turned around and came back, by then he was having seizures. Both these foals crashed quickly and despite supportive therapy (fluids, steriods, valium for the seizures) both were lost within a few hours.
Because your filly is hanging in there, hopefully this is something else and she'll pull though! Keep us updated!