I Am A 15 Year Old Girl (10th Grade) And

I am a 15 year old girl (10th grade) and have recently found a small, hard mass on the back of my head to the right near the hairline. I would say it feels about a centimeter in size and isn't squishy at all. It's painless to the touch and doesn't move. I haven't had any infections or illnesses recently. I feel exhausted a lot, even on the days I get 9 hours of sleep (on the weekends), but I think the fatigue is mostly due to my poor sleep habits during weekdays (usually 4-6 hrs a night). I found a small hard lump in the same spot when I was in the 5th grade and got it checked out by a doctor (because my mom was worried it might be lymphoma; both her and my grandpa had it), but he said to not worry about it because it was so small. I'm not sure if it's been here the whole time since then or it went away and came back, but it does feel slightly bigger. What could it be? Should I let my mom know? She would be worried about it if I did and would schedule a doctors visit right away. Should I wait and see? Should I be worried? Thank you for your time!