I Feel A Mild Dizziness When I Position My Head

I feel a mild dizziness when I position my head face up or when I woke up from bed. It only last few seconds, this started when I had accident with head injury. it was frequent for six months . after city scan and some medications I got healed. But recently it started the same head position laying and waking up just for few seconds. I guess lest than ten seconds and it will be over whether I change the position or not. Now tell me what you understand by this?