Hello! Thank you for the query. Every antibiotic can cause symptoms mentioned by you. Please note that healthy intestines have good bacteria on theirs surface which protect intestines from bad bacteria. When you take an antibiotic, good bacteria are being destroyed and bad bacteria can colonize intestines. This can cause many symptoms including constipation, abdominal pain, bloating, diarrhea. Prilosec decreases stomach acid secretion which is a kind of antibacterial barrier. Red stool can indicate blood presence. If the blood is on the surface of it, most probable source are hemorrhoid. If the blood is mixed with the stool, large intestine tumors should be considered. I suggest you to start probiotic supplementation and see if it will help. If not, stool tests and colonoscopy should be done. Hope this will help. Regards.