Well, I think there is a chance that Rain is nursing on Bonny. Bonny has a bit of a bag, I noticed it yesterday, when I was trimming her hooves, and it does have milk. I had them separated for 3 months, maybe it was 4 months; and while they were in adjoining corrals, Rain should not have been able to nurse.
If I can figure out a way to do it, I'm out of corrals, I may have to move either Rain or Bonny for longer so Bonny can dry up. No wonder she's not gained more needed weight, if she is indeed nursing her naughty daughter.