I have 4 mature female guppies and a lot more that i'm keeping that aren't ready yet. I dunno if your guppy has given birth b4 but if not, that might be why she not very fat.
Mine (and somebody else said this) get fatter all of sudden when their babies gonna come. if she is gonna give birth, then it's normal for her to dwell about for a while. Mine sit on the bottom for a day or two and then all of a sudden they're not fat anymore. Are you sure she hasn't already had the fry and you can't see them? the not eating and hiding happens with platties, I know that, so maybe the not eating is the same with guppies. I haven't noticed to be honest coz I got so many of them, continuously having fry. HTH. Is there a filter in the little tank? have you cleaned the poops from the bottom? the ammmonia in it makes the fish very sick, I had a lot die a while ago from that.