This is the skinny on acid blockers:

1) they reduce acid in the stomach by either of two ways... by blocking the proton pump (Prilosec/Prevacid etc) or by blocking the H2 receptor (Zantac/Pepcid etc).

2) Intrinsic factor needs acid to work and bind B12 from food.
So constant use of these acid blockers, or a natural decline in acid (which happens when we age) called achlorhydria... decreases the intrinsic factor ability. The normal amount in food daily of B12 is about 2micrograms or so, and intrinsic factor is necessary for such small amounts.

3) Also acid blocking drugs impair your ability to digest protein.
The protein normally is broken up by acid in the stomach. When this fails, due to acid being not present, the protein moves on into the intestines and colon and FERMENTS...creating gas, bloating and diarrhea. Your dairy problem may be this, and not lactose intolerance.

You can still take B12 in a massive dose 5mg a day on an empty stomach. You do not need intrinsic factor for these high doses, since at 5mg some will be absorbed PASSIVELY in the small intestine. The presence of food, will block this type of absorption.

You can still take your B12, but if you continue with acid blocking drugs, and you will continue with gas, bloating and all the rest of the GI disturbances from undigested protein in your food.

As far as the Kefir goes, try a small bit daily in the beginning. 1-2 ounces and you may get the benefit of the probiotics and not have the gas from the dairy protein that is in it. Some people find the whole fat version better in this regard. My local Kroger's only carries the low fat types. As you build up the new probiotics in the bowel, the gas may improve slowly so you can consume more. I tend to have 4 ounces-6ounces once a day. I have it as a dessert after dinner, or as a snack around 11am if I get up early and have early breakfast. I was having massive gas, bloating and nausea this summer and fall, and within 3 days of Kefir it was all gone! A quart is lasting me around 5-6 days at the rate I am drinking it now. But it is up to you.

Acid blocking drugs really upset the body in serious ways. It may take some time to get your bowel back to normal.