Will slipping off my diet once make me gain weight?

I guess I should of said life change rather than diet. Anyway, I was craving something sweet and ate a Starcrunch bar and two little peanut er cups. I'm a little nervous as I had a great workout today and felt good...I have no idea how I got so week..now i'm gonna go to sleep where it is going to set on my all night...ugg....can I gain weight?