The Mag Citrate griped you, that's all. You took way more than was required to do the job, especially for a first-time experience, and while "backed up", too. That's like killing a gnat with a sledgehammer.
You'll be fine.
Just drink water until it's coming out of your ears for 24 hours or so, while staying close to a bathroom. Then drop back to the "1/2 your body weight in ounces" amount after that, with 1/4 tsp of Celtic Sea Salt (Health food store) added to every liter.
IMHO Mag Citrate is OK if you use it in small amounts, say 2 ounces or so. I know you said you read the label, but sometimes you have to watch those labels :)..
BTW if you ever try Epsom Salts , another extremely effective laxative, stick to 1 slightly over-level *measuring tsp* in 16 oz. distilled water, chug it down, and that will work fine without this kind of problem.