Large water changes are to remove any potential remaining ich, so that when you dose again you're dealing with a smaller volume. It's how you treat most bacteria infections just incase you don't get all of it. It's probably either ich or another bacterial infection. Either way you should make the next water change after dosing a large one IME
There is a difference between ich and a bacterial infection though. Ich is not bacterial. It's a form of parasite. Antibiotics have low to no effects on ich. They are not classed as gram positive or gram negative. Bacteria is effected by antibiotics. They are a negative or positive gram based life. Raised temps will speed ich in its life cycle while in some cases raised temps have no effect on bacteria other then making them bloom better. Water changes also may not reduce any of the above.