The description should be more complete.all the symptoms and there duration should be conveyed.It is quite uncommon for an exclusively breast fed child to acquire intestinal infection and gastritis
The main symptoms of gastritis is vomiting
in such a child not the loose stools.lactose free milk should be ideally given after evaluation for stool lactose.
Breast milk is reasonably tolerated in mild to moderate lactose intolerance
currently in this situation best shot is stop all the drugs ,start breast feeding and then watch.Practice burping religiously.If loose motion
restart - get a fresh stool routine microscopy and lactose done.