It could be many things. Most common is water weight as alcohol is huge diuretic. Sort of like how MMA fighters can drop 10-20 lbs or more to make weight classes, then put it all back on within days from rehydrating. Also alcohol messes with your digestive tract making it absorb nutrients poorly.
Also alcohol can make you feel less hungry so you eat less.
I really wouldn't worry too much about it as staying sober is more important than that weight gain. Give it time, stick with your proper diet and exercise routine, and I'm sure you'll be where you want to be.