Gluten intolerance? does it relate to shakes?

 I went to see the GI doc because I need a preop endoscopy. I told him how I have been having very very bad diarrhea for about 4 months. Complete WATER!  I mentioned that I have a protein shake every morning because they are so easy and I noticed this is when it started. He then asked if I had ever been checked for a gluten allergy, which I had not. He then mentioned that a lot of people have problems with protein shakes. Are these 2 things related? Protein shakes don't have gluen in them, right?  Does anyone out there have a gluten intolerance, and if so, what type of symptoms do you have?