How can I get rid of this weight quickly?

I've always been really skinny, and I still am, but I have noticed in the past week I've gained a few pounds. I know why because my family has been buying junk food (we usually don't buy it) and I've been lazy all week. I really don't want to gain any more. I'm usually 110-112 pounds but I weighed myself today and I'm around 115. And my stomach is kind of chunky. (It is usually flat) How can I lose this weight quickly?