9 wk old has sm. lump behind ears & neck. 1 has gotten much bigger recently. no fever. dr. said lymph nodes & large one is cyst. i'm concerned the "cyst" grew so fast? worried its cancer or infection.

Concerning! : You are right. Rapid growth may suggest possible cancer or infection. In the absence of fever, cancer is a concern particularly if he has weight loss or night sweats. Lymphoma is a common cancer of the lymph nodes and can affect children as well as adults. Get checked soon to clarify the cause..

Nature of cyst: If the lymph node or cyst has grown and changed rapidly or is causing pain you should have your physician check it out again. You are appropriately concerned that this could be something more serious such as a tumor or infection. Look for signs of redness or infection in these "lumps". Infections are usually associated with fevers. An imaging study like an MRI might help.