Been seeing a chiropractor lately. He's been doing the Graston technique and neck adjustments on me. Today he tried a cranial adjustment where he pushed quite hard on my skull in various places. Immediately after I felt nauseous and dizzy. Had to lay down. I was worried that he may have given me another concussion. He said it was impossible because my head was stationary during the adjustment. He stated that a concussion could only occur from the head being shaken.

Back at home and feeling back to baseline now. No nausea or dizziness anymore. I guess my question is if it is possible to be given a concussion or even a subconcussive impact by having pressure applied onto your head?

Probably was the combination of therapies , and if you were nervous or already had concerns about the cranio treatment.. that could make you react more to it.
But no actual concussion would be caused by that therapy.

*If you had a skull/bone fracture of some sort very recently , the pressures could cause issues from that standpoint.*

But a knowledgeable treatment provider should verify any possible issues before treating..