I found a pea sized lump on the back of my 17 month old daughters head and im very concerned.

Reactive lymphnode: There are around 3 or less occipital lymph nodes found at the back of the head.They take in lymph fluid to purify it of harmful cells or particles. The lymph that passes through them is the one that has gone around the scalp. Since this is a tiny pealike, rubbery and mobile lump and your toddler is doing well-it will spontaneously resolve over a few weeks. Large, red, tender ones need a check!