In my neck is a peas-like pill, but it's completely painless. is it an enlarged lymph node that's thin under the skin?

Likely. Lymph nodes are scattered throughout the body but most noticeable where they lie atop muscles in areas where there is little or no fat or thickened tissue to hide them. The neck is one such site. They are generally pea sized at rest and enlarge when activated to cleanse debris from body serum.

You may feel the nodes every couple of weeks, if any of the lymph glands doubles in size, becomes painful, ulcerates, or other nodes become enlarged, or you develop fever, or lose weight, you should see a doctor. Wish you good health!