the complaints your child has may be due to some bacterial or amebic infection or secondary to transient lactose intolerance
that may develop after an episode of diarrhea
you should get your child examined by a pediatrician
and a stool routine with pH and microscopic examination and stool culture may be of help.
you can apply some coconut oil or some other lubricant in the area around anus an d get it evaluated if there is need of some anti fungal cream in that area of at sites of diaper rashes
your child may also need some antibiotics that can be prescribed only after proper history and examination of your child.
thankfully it does not seem to be a major issue with the child as she is fine otherwise.