How come my ankles and feet swell up after eating junk food?

Water retention: I'll assume by junk food you're referring at least to salty snack foods, like chips, or fast food like burgers and French fries and pizza. Where salt goes, water follows, so with salty foods we tend to retain water in our bodies. Gravity then draws the water to the dependent parts of our bodies, which would usually be the legs and feet. So they swell.