Yo-Yo water weight

When I am at work, I am on my feet for eight hours a day. I have lost 7 pounds in 6 weeks. My problem comes on my days off. If I have 2 days in a row off, I will gain up to 3# in water weight and it takes 2 work days to get it off. I dring around 6 cups (+/-) of water a day. I have been trying to cut down on my salt intake. I am usually within 100 cals of my 1300 a day. Any Ideas on how to help/stop this water retention? I seem to be stuk at the same weight for the last 2 weeks. Thanks.