Does Morphine Cause Gastritis?

I had been taking morphine sulphate 15 mg, once a day for three months with absolutely no problems. I am a very healthy, physically active person. I gorged on carmel popcorn labor day, a few days later I had horrible diarrhea and developed nausea, which I lasts for as long as four weeks. I normally do not eat junk food in my healthy diet. I will have periods of no nausea and am perfectly normal. I am watching for food triggers. The Gasterintologist said that I had gastritis and that it would take a long time to get rid of it. My research shows that morphine can cause gastritis, which concerns me, as morphine has given me back my normal life. If morphine is the cause, would taking bicarbonate of soda before or after cause adverse reactions . what do you think?