What Causes Sore Throat And Yellow/brown Mucus While Coughing?

Have had a sore throat for over a week with my voice being horse. mucus is yellow to brown when i cough. i have diffculty swallowing anything liquid or solid due to pain. went to the PCP with a sinus infection and they didnt seem concerned with my throat condition and prescribed a Z pak, Had heartburn before and diagnossed with GERD by the ENT and referd to the Gastro but havent went. what could it be and what should i do