1998 Oldsmobile Achieva Heater core?

My car, last evening while driving home from a nearby town. Started making steam and hissing noises. Soon to be followed by steam coming into my drivers compartment.(And leaving a stream of steam in a narrow straight band straight up the center of my interior windsheild). No warning light came on. And the heat guage was not overheated but withen upper normal range. I travelled probably a mile before pulling into my Garage. I shut off the car and found the steaming sound ended after 2-3 minutes. There was a watery smell in the air, and found a puddlle of antfreeze under the car. The next morning removed radiator reservoir cap. Found the level very low. And added a full gallon of anti-freeze. It began to immediately poor out just past the front wheel.( But a foot or more towards the center of undercarriage). It seemed to be pouring out from 4 ponts closely grouped in a seeming straight line. The radiator did not appear to be leaking at all. In fact had it flushed and thermostat replaced about 5 weeks ago. I had the motor turned off when adding coolant. Thought I'd add that bit of info. I had just been running the air conditioner just before the incident. I thought it might be the water pump? But when I phoned local garage, he suggested it sounded like heater core. This is the very first time I had these symptoms. And wondered what you all thought? I have a '98 Olds achieva SL, 6 cylinder motor. P.S. Had a mechanic flush radiator and replace thermostat one month ago. Did they somehow cause this?