I was put on antibiotics for symptoms of strep, but the throat culture came back 4 days later with a diff bacterial infection. what could it be?

Throat infection: This probably was a viral infection of the throat and the bacteria which grew in the culture were most likely only colonizing the throat rather than being an invasive infection. The throat infection should resolve without specific treatment after a few days. Your physician can advise you best, however as he/she knows all your findings.