benign positioning vertigo
Gents., I've had this twice. Go to YouTube and look at the recommended exercise, which is called the Epley Manoeuvre.
(Basically sit on edge of bed - turn head to 'bad' side - allow yourself to fall back in that position and hold for 30 seconds. Keep body still, turn head to other side, hold for 30 seconds. Then rotate whole body-and-head so head is positioned 45º face down to the floor, hold for 30 seconds).
I can testify that the exercise, which repositions tiny crystals within the semi-circular canals, is very effective. Although it is most unpleasant as your head whirls while the crystals take up the new position. But it works and is an almost instant cure.