What Causes Intense Pressure On The Back Bottom Of Skull?

I woke up this a.m. with intense pressure on the back bottom of my skull where it meets my neck. it eased without my doing anything and then a few hours later it returned. I took Tylenol the first time and then about 3 hours later, give or take, feeling returned so I took ibuprofen. its been maybe 3 hours since then. what could this be? a migraine?