Nodes that shrink with anti-biotics are not usually cancerous AFAIK
. Most likely they are swelling due to that UTI. But you may want to ask about auto-immune disorders too at your next checkup.
Meantime, try to relax enjoy your last month of no baby. It is stressful enough dealing with a newborn and the schedule upheaval that entails (I should know - we're expecting #4 in about a month ourselves).