How did I gain 5 pounds in like one day? And when will this go away?!?

Okay, so I'm a vegetarian and I wanted to be even more healthier than I already am so I oculd lose the last 12 pounds. Well, I gained 5 pounds in one day! ONE DAY!!!! I drank alot of water, because it is supposed to fill you up. I had salad and Wholly Guacamole for the dressing... half of the one hundred calorie pack. And I had like 7 pretzel sticks, 12 gold fish, and some vegetarian chilli that was about 500 calories all together. And then I jumped on the scale and see a that I gained 5 pounds. Now I am going crazy and I feel so fat! When will this weight go away??? They say to drink 8-10 glasses of water and I gain 5 pounds? What?! How is this possible? Help!!!