Using your heater wouldn't cause your expansion tank to explode, I think it was more a coincidence. Using your heater would lower the overall temperature in the system. Remember to change your "radiator" cap also if this wasn't done. This $8,- part is usually too blame for over pressure.

A busted or leaking heater core would manifest itself with either, smell in the cockpit, coolant on the floor, "steaming vents" as in humidity on the vents and windshield. Don't use the AC while testing for these symptoms. Usually this unit doesn't blow, but a hose connecting to it or something else in the system vents the pressure. With time they corrode, but it's to early for that..:)

Regards
Carl