Light-headed

For the second time in less than a week, I'm getting strangely light-headed. On both occasions I had been eating well, taken vitamin tablets, and experienced no other symptoms (nausea, cramps, etc.). I've been getting plenty of sleep (8 hours last night and the night before) and haven't been exerting myself too much, nor have I been particularly sluggish. I don't feel tired or lethargic, my head just feels very light. I have a tough time concentrating, I get dizzy easily and periodically stumble when walking or otherwise fall apart when doing something physical. And no, I haven't been drinking. Any ideas what the hell this could be?