whenever a lymph node enlarges in size it is either due to any defect in the node itself (like infection, cancer, etc.) of infection of the body part which the node drains i.e. parts of the body from where the lymph flown via channels to the particular node. From what you are stating, it is due to the latter. The infection must not have been treated completely the first time it occurred. I suggest you go for a complete regimen of antibiotics of 5 days and DO NOT stop the drug before 5 days even if everything seems fine, lest it might flare up later again.