is a big gain possible?

I think it's pretty difficult to do so that quickly. I had a similar experience and gained 9 lbs in a six day period around the super bowl. Although my calories were within my goals for that period, I had more carbs and sodium than usual. I think my body reacted to these different foods by storing some, but it appeared to be mostly water. I was able to resume my regular diet and exercise schedule and lost those 9 lbs in eight days.