I had a severe concussion six years ago, after skydiving three days ago, i've had a headache, dizziness and brain fog since. is it related?

See below: I wouldn't think, unless you hit your head when you were skydiving. I'd check with a neurologist if this goes on more than 24-48 hours or if it gets worse.

Skydiving: If you hit your head skydiving, it's possible, since after a concussion sustaining another one is more likely. It is best to see a neurologist or other physician specializing in concussion to help in assessment, since your symptoms would be consistent with a concussion.