Stomach bug, side effects of prilosec; anything worrisome?

Headache, diarrhea. If you're referring to headache and diarrhea, these are both very common Prilosec side effects that should resolve once the drug is discontinued. Many other acid blockers in the ppi class share these side effects. Worrisome symptoms however, unrelated to Prilosec but that mimic a "stomach bug" would include: bleeding, weight loss, persistent vomiting, signs of gut obstruction. See doc if concerned.