Laptop bottom warped

Hello,

I have a MacBook Pro Retina 2013 15" laptop. I've noticed that when on a flat surface that the four rubber feet don't all touch the table. The laptop can be rocked. Has anyone else had that experience? I'm thinking that maybe the battery is swelling and forcing the shell to bulge in the middle just a tad. Probably too expensive to replace the bottom shell and battery out of warranty?

Anyway, I thought I would throw that question out to the group.