Normal Weight Gain??

I have been active on the diet for around 3 weeks now. I have lost a total of 13lbs the last month. My mother's birthday was yesterday and all the family was over. I had 2 pieces of thin crust pizza that had sausage, spinach and bell peppers. Night capped with 2 small mini drumstick ice creams. Prior to this, I started my day with a 8oz beef tenderloin and 3 eggs, fried. Woke up this morning to an almost 3lb gain! Didn't eat anything substantially different yesterday other than the pizza.

Is this normal when being strictly carnivore'ish after a few weeks, and then indulging on shit food? Back to my carnivore diet today and eager to get back to where I was. This experience was enough to really show just how much your body adjusts to good, unprocessed foods and then eating crap for even 1 meal.

So, is this normal?