What causes hard knot like lumps on your skull under your scalp? doesnt hurt but i have been having headaches more often than usual. they are around the crown area of my head around about where i wear my ponytail?

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. These may be sebaceous/pilar cysts which are benign but can get infected. It would be prudent to see a doctor. Visit this site for info: https://my.clevelandclinic.org/health/diseases/23092-pilar-trichilemmal-cyst#symptoms-and-causes Wish you good health!