You might have benign positional vertigo due to an inner ear problem. You might go to an ENT for this. Do you have anorexia nervosa?
Welcome to the forum!
This could be due to postural hypotension (blood pressure falling with change of position), internal ear problems and cervical spondylitis or compression of cervical spinal nerves. All these cause dizziness when you get up from lying down position or when you change position that results in a change in the level of your head. Cervical spondylitis and ear problems also cause dizziness on changing the position of the neck.
It is difficult to comment without examination. Possible causes have been enumerated for you. You can discuss this with your doctor and get yourself examined.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!