Yeah I would say if you are worried you might have another issue, or even just to check as to whether you do or not in the mean time prior to changing your heater core.. bypass the heater core as shown above and run it for a bit once you have flushed the coolant.. and see if the dirt or rust shows up again... It does sound a bit more like rust that you are seeing in your coolant than dirt... I was a bit freaked out by the sight of dirt in my coolant too, only to find out it was rust from my radiator that was about to give way.