Could be bad. Have a poke around the back of the sill while lying (safely) under the car. Pull back the rubber mats where they bend up the inside of the rockers, inside the car. This will expose some holes or blanking grommets. Remove those and check inside as much as you can with a torch. If it's bad it will be obvious as soon as you peel back the rubber. Check the whole length of the rockers. The outer curved section is effectively a shaping/protecting layer. It's in 1 piece front to back and galvanised on the 4 door. The rocker on the 2 door is only galvanised from the front to the B post as the rear outer section is part of the rear wing. Rust in this rear section is common. The rocker is a 3 piece sandwich. Parts available but not an easy fix unless you're used to this sort of replacement. Peel back the rubber mats from as much floor as you can for rust inspection.