Milk cap sized lump on moms left back of head before hair line ends. what could this be, it really serious, does she have to have it looked at? hurts!

Yes: She should have it examined- by her primary or a general surgeon. Not sure if you meant within or outside hairline- but most lumps are benign- infected sebaceous cyst, trichilemmoma even a hematoma. However, it may be more serious- infection or malignancy and should be treated.

Lump: Yes it should be looked at if it hurts. It could be an abscess or a swollen cyst.