The probable reason might be BPPV( Begnin Paroxysmal positional vertigo).
Don't stand or sit at once. Do it step by step. For example, Before sleeping just sit a minute in bed, allow vertigo to settle and then sleep.the same applies when you wake up from the bed.
Next Steps
You can watch videos on youtube with a name called epleys maneuver. do exactly how it is done in the video. this is the best relief you can get.
Health Tips
if still vertigo persists, consult an ENT surgeon for further work up