My rat is really skinny does anyone no why?

She is abut 3 months old maybe a bit younger and she has gotten really thin she still eats her food and drinks water and runs around a bit (out of her cage and stuff) she also makes a little clicking noise (a bit different tho) that she never had b4 so does any1 no what it is ( i will be checking question every 5-10 mins if u have a question about her) thanks :D