Sugar = water weight gain or inflammation?

[AllyB]

I finished my whole30 and since then have noticed something.  If I have a day where I eat things with added sugar (not a ton, but certainly not whole30 compliant), I have a several pound weight gain the next morning.  I know I didn't eat enough calories to warrant gaining several pounds of fat over night.  So I'm wondering if it's water gain due to inflammation?  Or what is going on?  If I go back to being whole30 compliant, the couple pounds go away after a couple days.  I'm trying to figure out why my body is having this reaction and what it's trying to tell me...other than sugar is bad!