My Mother Is 95 Years Of Age And Had Been

My mother is 95 years of age and had been vomiting. It was brown in colour. She was sent to the hospital. She has had a CT Scan, a xray, blood test and urine test done. Vitals and blood pressure is good and has a bowel movement. The doctors could find nothing wrong with her and discharged from the hospital. The vomiting stopped while in the hospital as well as the following day. Now she has vomited once since then. What is wrong with her?