Well, for me, I know a lot of it is water, but the hormones also screw up my digestive system, and I always get a bit constipated when it's time for my period. I assume that's why it's SO much weight. I feel like the Michelin man though, and it feels soooo miserable. Then as soon as the cramps subside and the bloating goes away - I feel sooo skinny.
But if you're just gaining a few lbs, then it's absolutely most likely just water retention because of TOM, and it should go right down when you're over that hump of your cycle
Stay on plan, keep exercising and that extra weight will leave just as suddenly as it came