That could be caused by a swollen battery. I suggest that you open the laptop to inspect the battery. Follow the directions in the Service Manual. 

If it is swollen, it must be removed and taken for safe disposal. Be very cautious with it. 

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.