What Causes An Allergic Reaction Leading To Constant Sweating & Loss Of Appetite?

hello, my boyfriends daughter is about a year old and is experiencing what i thought earlier to be an allergic reaction but she is in the hospital and has a temperature of 103, constantly sweating, tender and in pain and worst of all she s been eating less and less He and i are worried sick about her as she is in the hospital right now with her biological mother all i want to know is what exactly would cause these symptoms in an infant please it is very urgent and not knowing is killing us both