Diet and exercise causing weight gain?

I am 8 weeks post Gastric Bypass laproscopic surgery.  I have began in the last three days to walk 1 1/2 to 2 1/4 miles a day.  My caloric intake is between 500-800 per day.  I take all vitamins, get in 70-90 grams of protein.  My liquid intake is poor but always has been, 40 ozs a day.

Since I started the walking I have gained two pounds.  I purchased a pedometer to count steps.  Frustrated!  Anyone found this to be true for them?  Any suggestions?  I feel I am following the rules to the T and back sliding.  Please Help!

Do stop weighing yourself daily. Once a week will be much better for your mental health. You are clearly on track, and your body will show the difference at its own pace. Since you didn't eat 2 pounds of food, you KNOW you didn't gain two pounds--you retained water. Maybe you're getting your period, maybe you ate something a little saltier than usual, maybe you're a consitpated, maybe your body isn't getting enough liquid and is hoarding--the possible causes are many and various.
 
Do keep working on your liquids. Have a second cup of decaf in the a.m. (there's an extra 8 ounces). If you do any liquid protein shakes (premade or that you make up at home, add an extra eight ounces of water (or decaf)--there's another 8 ounces.

Before giving up, remember that muscle weighs more than fat. See if your clothes fit better - it's a better indication if indeed you're building muscle at the same time you're losing. Also, remember to check in with your surgeon's nutritionist. Make sure, also, to get some fiber, even if it's the supplemental stuff that you need to drink. Good luck.