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Yes, stomach upset can potentially signal a serious reaction to Lipitor (atorvastatin), although such reactions are rare. The most common side effects of Lipitor include diarrhea, upset stomach, and muscle pain, according to the drug's label [1]. However, more severe symptoms such as severe stomach pain, nausea, and vomiting can be signs of a serious reaction [1].

It is important to note that Lipitor is a statin medication used to lower cholesterol levels, and while it is generally well-tolerated, it can cause rare but serious side effects, including liver damage, muscle damage, and kidney problems [1]. If you experience severe or persistent stomach upset or any other unusual symptoms while taking Lipitor, you should contact your healthcare provider immediately [1].

In addition, it is important to follow the dosage instructions provided by your healthcare provider and to report any changes in your health or any new symptoms to your healthcare provider [1]. You should also inform your healthcare provider about any other medications you are taking, as Lipitor can interact with certain drugs [1].

In summary, while stomach upset is a common side effect of Lipitor, severe or persistent stomach upset can be a sign of a serious reaction. If you experience any unusual symptoms while taking Lipitor, you should contact your healthcare provider immediately.

Sources:
[1] Lipitor (atorvastatin) tablets, for oral use. Full Prescribing Information. Pfizer Labs Div Pfizer Inc; 2016 [DrugPatentWatch.com] <https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/202080s036,202082s029,202084s029lbl.pdf>