Was there any sign of struggle in her little nest? If she was poisoned she would not have just fallen asleep in a favorite spot. Maybe she just died of natural causes? I think you would have noticed the distress in her eyes when you saw her the first time and said "hello" if she were poisoned.

I suddenly have more cats coming in. I keep two good mousers (spayed) but now there are five out there and one of them keeps pooping on my nice clean straw Ive put down in the foaling stall. I hate that!