Walaikum Asalam
Denter may not have completely remove the rust before repaint. Or your carpet for some reason is left moist or water maybe entering the cabin. These are the possibilities I can think of.
First get your vehicle checked if any water comes in during rain. Then go for repaint with anti rust primer. Also make sure before repaint the rust removed completely. You may apply spray based sealant after 90 days of repaint to the floor but that's just for additional protection. It may slow down the rust but not prevent from rust if water is coming under the carpet