What Causes Gastrointestinal Distress On Ingesting Amneal?

I have been taking Ranitidine 300Tabs for a long time to control gastric ulcers. I opened a new prescription, identified as manufactured by Amneal and had taken it four evenings including last night. I had unexplained gastrointestinal distress within one hour after ingesting which lasted about 3 hours. I had never had any reactions before and my gastric problems have been under control. I do not know who the manufacturer of previous prescriptions have been as I was under OptumRx s (United Health Care s Drug Plan.) I notice that Amneal does not even list ranitidine as one of its generic drugs yet OptumRx s bottle has your name on it. Do you make it? Has composition changed? What are the fillers? Why should I have gastric distress from a product that is designed to reduce gastric distress? Again, I have been symptom free until I started taking the new prescription bearing your name. I would appreciate your input.