This isn't a mag recommendation (FYI, I take 1000 mg/day in divided doses), but something else.
If you're having...uh, b'room problems, one simple solution would be to buy some Asher's sugar-free chocolates (any variety, but I like their English Toffee and their Vanilla Caramels). You'll probably want to factor in the cals so you don't go too off-plan, but it's not too unmanageable. Eat 3 or 4 of them in one enjoyable sitting.
That's all. Give yourself half a day, and you should find that your situation has changed materially.
Nice HIIT session!