Yes, taking large amounts of Tums or Rolaids can cause "rebound" heartburn. When I first started taking calcium supplements for osteopenia I used Tums because they were cheap. After a few weeks I started noticing that I had heartburn much more often. I actually wound up in the emergency room with severe back & shoulder pain. At first the concern was a possible heart attack, then gall bladder, finally they decided it was severe heartburn and sent me home on Zantac. One dose cured it. I switched to Caltrate for my supplement and have never had a repeat episode. Since most of the electrolyte drinks don't sit well with me, on longer rides I carry Elete tablets for salts, but I also carry a couple of Citrical Plus, which has magnesium added.