I had my FUE hair transplant in June of this year... so about 4 months ago.
After surgery I experienced severe pain in the back of my head, and when I researched it... information was surprisingly lacking.
The pain was as you said around an 8. It would get much more severe closer to bed time, but during the day time the pain was like a 3 or 4. Sleeping was brutal, I had to sleep on a inflatable J-shaped pillow sandwiched between another pillow in such a way that the back of my head was "floating" to reduce the amount of contact the back of my head had.
Everything I did read said that experiencing such pain was extremely rare and only occurred in a small small percentage of people. That upset me because I felt a bit alone and couldn't believe it was "that rare" because I regard myself as a fairly healthy person and I went to one of the top doctors in the world in Turkey.
Anyways, the pain does go away by day 14. Around day 10, the pain did not subside at all and I was starting to panic because I read a situation where someone had pain for several months and years and had to go on Lyrica for nerve pain.
Basically the nerves in the back have to regenerate, and during that process, it causes severe pain. It isn't an infection. I could go more into detail on how the nerves can spiral into a coil like formation and what happens during nerve regeneration but this message is getting too long as is. Please take lots of B vitamins as vitamin B6 helps with nerve regeneration/health.
I used Ibuprofen sparingly because it hurt my stomach as well, and lots of aloe vera.
By day 14, I was 90% better... just wait 4 more days and tell me how you feel.
It was seriously an emotional and painful experience after and I really empathize with you, especially because this is one of the least talked about subjects of hair surgery and literature makes you feel "abnormal" and "rare"... You will be ok, just a few more days!