I realised my 11+ yo has a bump on the back of her head, like something under the skin

I totally understand that worried feeling when you suddenly notice something on your child that wasn't there before. My younger one had a small lump behind her ear once and I panicked, but our PD checked it and said it was just a swollen lymph node that settled on its own after a few weeks. For something like a bump under the skin on the head, it is really worth bringing her to see your PD just to get it properly checked, because they can tell you whether it is something harmless or needs further attention. It is always better to have peace of mind than to wonder, and your PD may refer you on if needed. Hope everything turns out fine for your girl, hang in there!