Juvenile idiopathic arthritis is not commonly associated with a skin rash, except in one sub category called systemic arthritis where the child has high spiking fever with a rash that comes and goes. As you have not described any fever, I think that the skin and joints may be different problems. She could have had an allergy to a new drug/ an unidentified trigger that gave her the hives. If the bumps on her skin are persistent she may need a punch biopsy to check the nature of the inflammation. The stomach problem may again be linked with arthritis or reflux. This is a problem that can be best addressed by a peadiatric rheumatologist, and you should meet one local to your residence. This is important to tie in all three complaints: joints, skin and stomach.