I'll tell you how: it's water weight. Physiologically, it's impossible to gain five pounds of fat tissue in less than 24 hours unless you overate 17,500 calories.
I assume you're a female of childbearing age. We're very prone to wild fluctuations in water weight; hence, I only weighed myself once monthly about a week after the 1st day of my period. Weighing myself more often was a blow to my sanity, so I didn't do it.