Same thing happened to me. I kept reading about how important it is to drink water so I committed myself to drinking my 8 glasses a day. After a week I had gained a little over 5 lbs, when I stopped drinking so much the weight went right back down. I'm not sure if it was the water or something else, but like you it completely freaks me out seeing the numbers on the scale rise when I'm doing everything right. I quit my strict water intake after about 2 weeks, and now drink a little more than I used to but still less than I was drinking. I'm interested to see if this is common and if there is anything to do about the water weight too.