I have 5 foster kittens from the same litter. They're around 3 months or so. One kitten never seemed particularly small, but in the last 2 or 3 weeks I've noticed that the other kittens are getting bigger while she is smaller, thinner and bonier. She eats the same as the others. I've just started trying to fatten her up a bit by giving her extra cat-calorie paste and snacks. I am starting to be worried about her. Though she seems to be energetic, she looks slightly sickly to me. Do you think there's anything to be concerned about?