Excess magnesium can cause diarrhea, but I suspect the initial electrolyte imbalance should resolve within a day. There's probably something else at work here. Prolonged diarrhea can cause magnesium deficiency, so I would continue supplementing magnesium. That might be off-putting because of the perceived connection with the diarrhea, but magnesium is an essential nutrient and electrolyte. Diarrhea flushes some electrolytes, and people die from electrolyte imbalances caused by diarrhea. (If you take low doses of chelated magnesium, like 1x200 mg once or twice a day with food, it should not make the diarrhea worse.)

You should also make sure you're consuming sodium and potassium, preferably from food. Drinking large quantities of pure/tap water without also consuming sodium and potassium will make things worse. Sodium should be taken with carbohydrates/sugar to be absorbed properly, though this probably doesn't matter if you're getting enough sodium from diet (which most people do).

Speculating, but it could be that the initial diarrhea caused some inflammation or disruption of gut flora. Could have helped (or simply been coincident) with an infection (gastroenteritis), in which case you'd probably notice some other symptoms like fever and nausea, but not necessarily.

If there's any blood, or unusually sharp or intense pain you wouldn't normally associate with bloating or diarrhea, or if you feel short of breath or like you're about to lose consciousness without standing up, see a doctor ASAP. You should probably also see a doctor if this diarrhea lasts much longer.

So you know, there are drugs (mainly loperamide, not sure about the brand name in your country) that are available over-the-counter in some states that can temporary suppress diarrhea/cause constipation, but I am not sure if it's advised to use them when the cause of the diarrhea is unknown, and might be an infection.