Hello, most likely you have been prescribed an antibiotic called azithromycin. It is used commonly for treating acne. Diarrhoea is a known side effect of this medicine as it increases the gut motility and/or sometimes kills the good bacteria present in the gut- in this latter situation Probiotics will help counter this problem. Azithromycin is unlikely to cause stomach ulcer. Is the diagnosis of ulcer made by you or by your Dr? If the current medicines are not helping you, then you can contact your doctor again for an alternative medicine or visit a MD Physician for stomach pain. Hope this helps, Dr Pinanky Jadhav, Pune