Why do I have PMS for 2 weeks?

I gain like 6 pounds during PMS, and I get bloated and bad trips to the bathroom, with pimple breakouts. This is all two weeks before my period. Then during the week of my period I lose the weight and everyone says I look good and fit, and still have a little bloat but other than that I'm ok. Then after my period I'm the happiest and most confident. Only for one week. :( then PMS starts again.

I I researched this and I heard magnesium can help. So I bought magnesium and I take them everyday. (250mg - 500mg)

and end I exercise nearly every day and eat healthy. I eat chips but it's usually unsalted potato chips (only ingredients are potato and canola oil)

i eat every 3 hours and eat around 1500 cal a day. I'm 15, 5'3 125lb
Why do I have PMS for 2 weeks?
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