I recently started cardio in the gym, lost 5 pounds and my butt got smaller. If I gain 5 pounds will it go back to normal?

The fat cells in your buttock that were transferred shrink when you lose weightif you gain weight it will come backGet a Fitbit watch and track your calories when working out.  Simply eat more calories than you burn off and you will gain weight.  It it is well documented that weight loss is due to dietary restriction and not simply exercisehope this helpsdr Del vecchio 

Hello dear!
Thanks for the question and provided information as well.
The answer is yes, if you gain the 5 pounds, you will get your previous buttocks' size back.
If you have any concerns, I suggest you to follow up close with your plastic surgeon.
Good luck :)

Weight gain will likely mean that your buttock will get bigger if you found that you lost size there when you lost weight.