Feeling dizzy do the point of blackness?

I have no idea what's going on. For an year now I have been feeling really dizzy. Every time I sit down, the world starts spinning. I stand up and vision spins and my surroundings blur to darkness. I have to wait a few seconds before I can clearly see and then move. It even happens when I am walking, so I look like a drunk woman flailing around. I checked my blood pressure it's usually 90/60. I went to the doctor and she wasn't sure what was causing it, since there can be many.

My my diet is normal, so I am not sure why this happens.
Feeling dizzy do the point of blackness?
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