I have had this sensation twice, once with a bad headache only on the LEFT side of my head, and once with no headache which left quickly. Also have had a couple friends feel this.
The time I had the headache and the times my friends felt the pressure we were on Yellow Butterflies and Yellow Gators if it matters, both strong pills. The headache-less time I was on an RN.
It totally scared me the first time, which was Halloween. My head was aching, the pressure was bad, and I felt cracked out and like I wanted to puke for about a good 5 hrs, then just cracked out and wired for another 3 hrs. Def. a bad roll. I was not dehydrated.
The main reason I think it happened to me was sinus. My left side of my sinuses as well as my left ear get sinus infections pretty often, and I also get migraines but only usually on the left side of my head. I think I was having sinus issues at the time and I just FELT the pressure more, not that there really WAS more pressure.
I might be wrong, but I seemed to get the pressue feeling only when I had an infected or irritated sinus.