I’m betting it’s more likely to be the antibiotic that’s bothering her. Mylanta generally will cause loose stools if given too often or too much, but I ever read here that anyone’s has seen increased reflux from it.
I saw your other post too, and tomatoes (so ketchup) is often a big problem for babies, not just reflux babies. I used to babysit a little boy who’d be miserable all day if his mom (who was nursing and pumping) ate tomatoes. He didn’t have reflux but he’s spit up a lot on those days and he’d be especially irritable. My babies used to react to tomatoes too, although not as bad as this little boy did.