When you workout (harder or more than usual), your muscle fibers tear and retain water to help repair themselves. This can cause a gain on the scale. It is temporary. Drink lots of water! Plus pizza does have alot of sodiom which can also cause water retention. Our bodies are made up of mostly water, so our weight can change minute to minute based on many factors (food and drinks we take in during the day, sodiom, etc). Plus, the scale is only one way to measure success. Losing inches and body fat is another! I would much rather be 150lbs and a size 4 than 140lbs and a size 6. Keep working out! Your doing amazing!