My Holland Lop litter of 7 bunnies is 4 days old. A few of them are smaller than the rest but only one, probably the smallest one, has a shrunken belly. It is very weak and the mother has not been cleaning its bottom very well - it is wet and dirty. I tried restraining the doe and letting that one eat, but she wouldn't relax and the baby was too weak to try for long. I've never had this happen before. I have no other litters available to foster to, and I'm scared to try a formula. What to do?