its a lot of stress to be taken away from a mother and put into a stall when you're not used to it. The soft stool and changes in eating are normal. There is no good reason she needs to be back with mom either, we wean ours at 6 months, we put them in the trailer with mom, take them to another farm and put them out in the paddock, then load mom up and leave the baby there. Also we don't feed the foals a lot of grain, mostly just hay. Grain gives them too much engergy and they can get too hot, epecially the sweet feed.