Why does my 1997 Chevy Lumina stop overheating when I use my heat

The engine radiator is performing marginally and the heat disbursed by the heater core makes the difference...live in S. Hemisphere?

I recently replaced the heater core no I live in the midwest I'm being told it could be a stuck thermostat or water pump

Ok, just wondered that because we usually don't get many heater core questions in June. Yes, overheating can be caused by a bad thermostat or water pump, you were advised correctly. I also stand by that running the heater will help cool engine. Check coolant level too. You may want to consider having the system pressure tested, if you have leaks. Or system was not purged when core was replaced.

I have it at my husbands family's shop right now but just wondering what I'm about to hear tomorrow hoping it's a thermostat that's stuck :/

Me too. That's the least expensive, except purging the system and that one is a longshot anyway