A Week Ago I Had Terrible Burning In Upper Abdomen

A week ago I had terrible burning in upper abdomen and watery stools along with vomiting. The vomiting was on and off for one day, watery diarrhea was 3 days. A week later, watery diarrhea has returned so far for three days along with stomach pain and a red itchy rash on upper arms, side of torso and palms get red and itchy.
Went to ER yesterday b/c stomach pain was bad. They gave me IV fluids, did chest X-ray, abdominal CAT scan w/contrast and took a stool sample. Everything looked fine. ER Dr said gastritis but also mentioned colitis. There is no blood in stool and I don’t have a fever.
I do have a history of acid reflux and IBS. Any idea what I’m dealing with?