From http://www.bbc.co.uk/health/conditions/vertigo1.shtml#benign_positional_vertigo:

Benign positional vertigo
For some people, the recurrent attacks of vertigo may be short-lived. The most likely cause is a condition called benign positional vertigo. This tends to affect older people and any sudden movement of the head (usually head turning) brings on acute vertigo without warning. Although it tends to last under a minute, it can be very unpleasant and disabling.

As with other causes of vertigo, this type usually follows a viral infection or is a result of inflammation or damage to the middle ear. Fortunately, it usually settles after a few months without treatment. Specialist physiotherapy can benefit some sufferers if the symptoms persist.

A more doctor-focussed explanation is at: http://www.patient.co.uk/showdoc/40002345/

Both say it should get better of its own accord in a few months, if this is what it is.
(I give out this sort of info as part of my day job).