Depending on the antibiotics, you may have cured it if you had something, but for sure, you could be getting false negatives.
Over how long have you been taking the antibiotics? Has this been an ongoing issue? So for example, I just went through a whole tooth thing where for months, I was battling a tooth infection, and was on 3 rounds of antibiotics over a 7 month period. Is that similar for you? I ask because even if the antibiotics haven't cured you, they can slow the growth of something to the point where you aren't seeing big and bad symptoms.
After you are done this round of antibiotics, you can wait 2 weeks, and test again.