Digestive issues are pretty tricky, and you really have to pinpoint exactly what is causing them. Because they are so individualized, it is pretty much impossible to predict whether protein powder will cause stomach upset for any specific person.
When it comes to protein powder, many cases of upset stomach come from whey, and more specifically the lactose that whey contains. For customers with lactose intolerance, we offer blends with whey protein isolate as the primary ingredient. Isolate is very finely filtered and is virtually lactose-free. If you still experience issues with the small amount of lactose in whey isolate, we also offer 100% plant-based blends (organic pea protein and organic brown rice protein) that are hypoallergenic and easily digestible.
Overall, trial and error remains the best method of figuring out what works for you and your stomach.