Muscles holding onto water....but yeah that would be a ****** to see on the scale!!! Drink some milk right after your workout, and maybe part of a protein shake and up your water.
Not cool, but it’s not fat. So be encouraged that an 8lb gain in one week is most likely water, unless you decided to eat 5 cheesecakes all week long. 😊 it’ll come off as fast as it went on.