Suggest Treatment For Swollen Lymph Nodes In The Neck

Hello! I am a 25 year old generally healthy female. My question is about the swollen glands in my neck. They had been swollen about 9 months before I went to the doctor. I had only noticed the gland on the right side being swollen but the doctor said the other was slightly swollen as well. She put me on Bactrum BID X 10 days. Today is day number 3 of taking the med and I have not noticed any difference. I am very scared of what this could be. My Dr. said that if the antibiotic does not clear them up she will need to do a biopsy. What do you think is the most likey cause of this? I do work in a hospital.