Were you taking electrolytes? You might be retaining water due to an increase in salt. You’ll lose it and more. I’ve often been at massive calorie deficits without losing anything for days and then all of a sudden I lose 5 lbs.
Is it Inflammation? Depending on how you trained and the stress you/your body are under you could spike your cortisol and or just retain water in your muscles. Heck after a leg day I am usually up 3+lbs.