Its possible it could be meniers disease, this comes and goes at odd times and can affect you when walking, sitting or upon suddenhead turning movement, when you are sitting at a traffic intersection and vehicles are passing across the front of you do you experience small moments of giddyness and have to stare straight ahead, or if you move your head quickly and it happens.
Then it could be meniers disease, its a strange condition that may have signs of tinitus or hearing loss, or even sounds become tin like in quality, sometimes with headaches involved that run from your neck up over one side of the head, or even of migraine intensity.
It may be due to fluid movement or pressure in the inner ear, or even the herpes simplex virus (which we all carry) becoming active through lowered body resistance due to illness or injury.
Anyway, do have a read up on this and see if its in any way possibly your root cause.
I often find that to just, stand or sit, and look at a close object for a short while, then the symptoms aleviate, also sit down if possible and gently tilt your head backwards and slowly from side to side, and I do mean gently.
Good luck with it.