Mylanta is notorious for giving the runs, I use a mylanta supreme from America which doesn't cause this problem for us, but you could try gaviscon or gastrogel, both are antacids and both will work in a similar way to the mylanta.

The body may also be adjusting to it, and may just need some time. How much are you giving? and how often.

Also it is best to space the zantac and the antacid for at least an hour, longer if possible as it may interact and make the zantac ineffective.

The medicine that will be helping with the reflux the most in the long run will be the zantac this should be given like a course same times every day, and the mylanta given when needed, this will work more instantly while the zantac may take a little while to get in the system.