Whose bump mystery?Whose bump mystery?We spy with our showbiz eye...

An unusual belly bump.

This svelte lady's stomach is usually as flat as a pancake.

Reese Witherspoon is the owner of a newly sticky-out tum, which she's parading at an Avon Walk For Breast Cancer event in San FranciscoReese Witherspoon Reese Witherspoon  yesterday.

So what's behind this strange swelling?

Fear not, Jake Gyllenhaal lovers, we're not suggesting Reese and her man have been up to no good.

It could be just a bad angle, but we suspect the slight protuberance under Reese's threads may be the result of a date with these tasty-looking cupcakes (right).

We'll keep you posted on whether Reese's appetite grows...