I too had a painless swollen lymph node that seem to decrease with antibiotics. I had 2 rounds of antibiotics,blood tests, X-rays, CAT scan, fine needle biopsy, and a scope all were inconclusive, the blood test showed nothing to be alarmed about. It wasn't until they put me under to do a frozen biopsy the surgeon discovered (visually, before the biopsy result were back) stage IV tonsil cancer with mets. I still wonder how the surgeon could tell me it was cancer by looking at it and all the other test were inconclusive. Granted its very rare in my age, I just don't think they were thinking cancer.
But that being said, all my tests were inconclusive, knowing now I would have pushed harder for an answer instead of waiting patiently for the next test to be done as months slipped by.
I guess what I am saying is until you have an answer keep asking.