Why would protein shakes (no matter what variety or brand) cause stomach aches?

Protein shake: Sometimes it's not the shakes, but the guzzling all at once. You can try half concentration to see if that helps. You can also try half the dose in half the water but taking it twice per day.

Dr Ali and another doctor agreed

Food Sensitivity: Many protein shakes on the market are made from whey, casein and/or soy proteins. These proteins, which either come from milk or soy, are highly allergenic and many people have trouble digesting them or tolerating them. I would suggest avoiding them since they clearly cause discomfort.

Dr Satulovsky agreed