Have you ever seen a migraine specialist? This seems like classic migraine associated vertigo. Dizziness appears suddenly after a migraine attack. This doesn't have to be what you would traditionally think of as vertigo or "spinning". There are receptors in your inner ear for rotational acceleration and also for linear acceleration. Any of them can get screwed up (or have their signals to the brain get screwed up).

Some people respond to migraine treatment (medication). Try looking at stories on mvertigo.org to see if any are relatable.

Mark is right that it could also be an unresolved neck injury. Just the fact that it came on after a migraine attach makes me suspicious for migraine.