You might be retaining water. Nausea and vomiting are caused by the magnesium citrate. Make sure you see a pcp, they'll be able to do something for you. Your medicine could also be causing it.

Go back to doctor

You need to see a gastroenterologist. You may need an Endoscopy to see what is going on in your stomach and esophagus. Could be inflammation, gastritis. They will check you for H Pylori. Ulcer. It’s not a big deal and you need to watch your diet and see what is the culprit. Maybe you are lactose intolerant, cut out milk products ice cream, cheese to see if it gets better. No soda or seltzer. No caffeine have decaf coffee. No cold cuts. No artificial sweeteners no seed oils. Some veggies might do it like broccoli beans. Cabbage. Berries seem to be ok. I can eat only pecans other nuts are a problem.