My step brother who is 5 was referred for something similar last year, he had to go to a speaclist and had bloods and scans and all came back fine, he was cleared of everything in the end and they eventually went back down, his bloods had also come back clear With children they don't like to wait and see, the blood tests will help to see if there is any viral or bacterial infection causing them to be swollen, and recently my daughter had lumps in the center of her throat that she was referred on to have a scan for and hers came back as cysts, again nothing to be concerned about, I hope everything goes OK x 

My step brothers was really large, we actually thought the worst, he seemed fine in himself but his lymph nodes where very noticeable, but thankfully all clear. And my daughter started with one smaller 1 pea sized then 2 more appeared near by id say the size of a marrow pea  but again thankfully its cysts which apparently is common. I hope they get the tests done soon so you can get some answers, as no matrer how much you try not to worry your going to as its your baby x