It is probably mostly water, but it is also your body rebelling - that was a very unhealthy evening. If you had been making better choices for weeks, and all of a sudden you go out of control with calories, carbs, and alcohol...your body is upset.
Drinking more water, eating better, and exercising on a regular basis will help you get that 5 lbs off again, and I don't want to sound like your mother or anything....but next time you might be happier with yourself if you go for a little splurge, rather than off the deep end!
Take care - things will come around. This is a journey to long term healthy habits. We all slip sometimes.