Rectal prolapse can occur in adults as well, but in children, it is usually common below the age of 3yrs, many occurring about the age of 1yr. Most in children also resolve without the need for surgery except if neglected or severe.
So dear mum, do not be overly worried, but pls do take her to be properly examined and treated"

Kindly look into her diet, if her stools are hard, give fruits and vegetables to help it pass easily. If having diarrhea, let the cause be treated. Till you see the Doctor, gently push it back in if it comes out (that is if it stays out/does not return on its own). Further treatment will depend on what is found when it is examined.

It may be a Rectal prolapse or haemorrhoids.
"Now, what can cause it? A number of questions need to be asked- are her stools hard that requires her pushing so hard to pass them?Has she been having diarrhoea (frequent watery stools) ? These can predispose to a protrusion. Does the protrusion return into the anus by itself or do you have to push it back? Are there any blood stains in the stool or after passing stool?
Please take her to be examined by a Paediatrician or Paediatric Surgeon.