Just notice a knot behind my right ear. What could it be from? Is sore.

Lymph node. There are thousands of lymph node located around the body that help screen out germs in the body serum before it returns to the circulation. There are many in the neck, head ; behind the ear. At rest these are often 1/4 inch in diameter and may grow to marble sized when reacting to a stimulus. Bug bites, or viral infections can increase their size.They should shrink down to nl size over a few wks.