Hi aussiegirl,
How old is she? You mention she feels thin - could you weigh her on a set of kitchen scales to get an accurate idea of her weight? Ideally try to weigh daily for the next few days to see if there is a pattern of weight loss.
When you say she has an upset stomach, do you mean diarrhoea? How long has she had this for? Does she eat plenty of hay?
Are their any other symptoms with the hair loss - does the hair pull out easily when gently tugged, does she scratch a lot?
Given all her symptoms I'd tentatively suspect a fungal problem, however more information is needed and myself and other members will be able to provide more useful ideas once we have a better idea of her problem.
I have to say though, as always, you should also seek the advice of a skilled and experienced vet.