If this problem has been there for a long time and she is not growing well for her age, it well fits into chronic diarrhea
and has to be investigated for various intolerances and malabsorption
A systematic approach will be needed. She has to be examined fully and some blood and stool tests will be needed.
Till then try to reduce the fibre content in food. Give her plenty of yoghurt (not flavoured or the sweetened one). Give her rice based items, boiled potato, porridge made of sago/ rice etc, soy milk, chicken puree etc.
Avoid fruit juices, aerated drinks, cow`s milk and formula milk.
For her rash
in her bums- use barrier creams like siloderm. Do not wipe. Just mop the area gently with wet cloth. Try to keep her off nappies for maximum part of the day.
If it does not heal, it will be very painful for her.
Approach your Pediatrician
for a zinc supplement
Hope my answer helps you.
Wish your daughter good health.