its jus a normal anti-biotice doesnt necessarily comes for STDS only so CHILLAX and find a new Dr
Sometimes doctors over prescribe antibiotics. He may of thought you had a infection that is not common and prescribed you the mess as precaution.
I don't understand either why they gave you meds if your cultures came back negative. Maybe you have just a regular throat infection? Give them a call today to ask why you were prescribed meds.