why does TTOTM cause weight gain?

So people seem to gain weight before their period starts but is it normal that i do after it has started? I made the mistake of weighing myself on the second day, and lo and behold i had gained 2.2lbs. What causes this weight gain? Is it water weight and if so, who does your body retain it during this time? Fingers crossed, it should disappear when my period finishes as long as i continue to eat well, exercise and drink lots of water right?

On your last sentence: correct.

You retain water around the time of your period. That's the cause - general bloating, and water weight.

http://www.mayoclinic.com/health/water-retent ion/WO00130