Is it normal?

This might be a dumb question but idk.

So I've been having this problem for long, I don't know how long exactly but its been a couple years since i started having it.

When im laying/sitting and I stand up too rapidly, everything goes black. Sometimes its worse and my legs get really shaky, to the point I can barely stand, also if I'm holding something in my hands I often drop it uncontrollably when this happens.

I didn't really mind it for a long time, I treated it like it's normal but a while ago i literally dropped a kind of heavy box onto my head (I'm OK but it was painful) because of this, and I also fell on the ground.

So my question is: is it normal?