Go back to your doctor as soon as you can and if i were you don't give her any more. Loss of appetite can be a side a side effect of Ritalin(happened to my son initially).The fits are a less common side effect. Although this is the only one she has been offered so far there are alternatives. Ritalin has methylphenidate base but there are others which have a dexamphetamine base which can be used if the other one is not suitable. It sounds like she is having a bad reaction to them-is she also restless, irritable, disturbed sleep, tearfulness for no reason?