My Husband Has Nueroendricrine Cancer. He Also Has Stomach Motility

My husband has Nueroendricrine cancer. He also has stomach motility issues. He has daily 4 to 6 times day diarrhea. He is on Reglan, and acid reflux med and Gastroenterologist presribed Xifaxan last Wed. for possible infection. Since sat. has had increasing stiffness joint pain and muscle pain. Stiff neck also. Very tired, lethargic. Is this just a side effect or something more concerning? Thank you