A woman thought she heard fireworks near her Brooklyn home Sunday night, only to discover that the sound was bullets — and her 19-year-old grandson had been shot, police said.

Brenda Robertson, 56, was inside her Brownsville apartment at 300 Stone Ave. at about 8:30 p.m. when she heard two gunshots, police sources said.

She ran downstairs and found the teen, Rashasheed Stallworth, bleeding profusely from his neck and surrounded by other people, police sources said. The bystanders flagged down a passing NYPD patrol unit.

Police observed a small blood trail from the Howard Playground across the street from the building, the sources said.

There were no arrests, but the shooter was described as a man wearing a black hoodie and black pants, cops said.

Stallworth was taken to Brookdale University Hospital and underwent emergency surgery, the sources said.