First responders rushed to Valerie Fairman initially thinking she might have suffered a heart attack ... but once on scene there were signs of possible drug abuse.
TMZ got the dispatch call to the fire dept and EMS Wednesday night in Coatesville, PA ... and the call went out as a "cardiac arrest" -- but about 10 minutes later the dispatcher changes the call to a "possible DOA."
She also says paramedics are going to need containers for needles. As we've reported ... it appears the "16 and Pregnant" star died of an overdose. She'd battled drug addiction for many years.