Aw, I'd definitely check this with your pediatrician soon, mama, bumps on the back of the head can be so many things (sometimes just normal baby bumps, sometimes fluid, sometimes bone stuff), pero kailangan ng actual exam para malaman kung ano talaga at kung kailangan ng treatment. Better safe than sorry, lalo na kung biglang lumaki or if baby seems uncomfortable, kaya go have it checked out ASAP.