Welcome! My name is***** am a UC Davis graduate, and currently, a Medical Director of a veterinary hospital.
I am sorry to hear about this concern. It is actually possible for a mouse to even cause an obstruction in the intestinal tract, so I would advise against waiting too long on a vet visit. Many times, I think an abdominal x-ray is well worthwhile, to see if the mouse is passing through the digestive tract or not.
Otherwise, you don't really want to give immodium, which will just slow down the intestinal tract and really cover up the problem.
In addition, not so sure your vet would want you giving pepto bismol either.
You can feed a bland diet, but if diarrhea persists, the vet will want to start some other medication for diarrhea and consider those abdominal x-rays.
Here is a bland diet recommendation:
Boiled boneless, skinless chicken
breast OR low-fat cottage cheese
Cooked white rice
*Never add on salt, pepper, oils, butter to any of the above
*Ideally, give 1/3 chicken or cottage cheese, and 2/3 white rice
Veterinarians will often prescribe some prescription bland diets as an easy alternative including Science Diet I/D formula.
Also, if Roxanne at any point looses her appetite or tries to vomit, then my concern is greater for obstruction.