About Phentermine and B12 shot

Hi,
My name is Lydia.  I started taking phentermine last week and took my first shot of b12 which imma start taking once a month.  Well i've been watchin my weight.  I exercise almst 7 days a week for 2 hrs sometimes.  And been watching my calories and eating healthy and less.  I took in the bad habit to weight myself each time after the gym.  My weight started at 218 and it dropped to 208 in 2days but each time i weight myself my weight goes up and down.  Is that normal?  I feel like im doing everything im suppose to do and i get discourage when I lose a few pounds and check again and see that it went up, well not back up to 218 but i feel llike im doing something wrong.