This happens due to changing pressure
in the canals of internal ear. However, if you are feeling nausea and
vomiting on changing your position, it can be due to postural hypotension (fall in BP while standing or sitting up from lying down position), compression of cervical spinal nerves, or due to internal ear problems.
Hope this helps. I think you should discuss these possibilities with your doctor. It is difficult to comment beyond this without examining.