I get them every now and then as well. It could very well be a number of things. Of course the best and 1st thing you should do is go to a doctor. But personally for me it was only because I was croutching too much infront of the screen and my ceiling fan was blwonig cold air to my back, so my shoulders got stiff. After some stretching of my back and neck and fresh air I felt better. I guess it has something to do with restriction of bloodflow to the brain or something... Dunno.
What I personally hate are those weird days I have where I think I've forgotten something, try bloody hard to remember it, or at the same time feel a constant sense of deja vu, about everything. Then at one moment I realises what I forgot and I get this major flash of thoughts and memories like I've suddenly become all knowing. Its euphoric almost, lasts a few seconds, then I get really dizzy and my ears ring and I have to sit down. And I'm in a total cold sweat. Hasn't happened now for a while now, hopefully not for a long time after... Hmm...
Anyway... Fresh air, no medicine like it.