Doubt its a heater core b/c of the timing- just too coincidental. Could be condensation if the drain hose has issues. Also you may have gotten some water into the A/C system if you drove through some deep water- if excessive it could get onto the floor.
Also check the areas where the wiring and hoses come through the firewall, esp where the EGI cables enter to attach to the ECU. Sometimes ppl run wiring or hoses for boost controllers through there and dont seal the area as well as they should.