Behavior in a nursing cat?

We found this kitten one night, it was obviously lost so we took it home. It was a little female. About a week later we found the owner and she came and picked her up. It was a month later we discovered that the woman had moved and left the cat behind. So we went and got her. Now I don't know why, well I do...anyways.. Not knowing about female cats she went into her "time" acting weird and crazy. Well she slipped out one night, came back the next day, 2 monthes later she's a mom. Now that she has had her kittens and is in somewhat "normal" state, she looks really scrawny. To me, if I can feel her backbone bumpy things she's a little too skinny. She eats like she has never had a meal before in her life. Any noise in the direction of the kitchen and she is moving heaven and earth to get in there. Meowing away and underfoot. Does nursing take THAT much out of them? Or could she be sick but we just never had her long enough or in a normal state to be able to tell? Her coat looks dingy too!