What Causes Dizziness And Headache?

every couple of weeks, I have sporadic dizzy spells. I have some kind of deja vu kind of thing, then I get really dizzy, tired, headache, stomach ache, and have trouble functioning. then after the spell is over I am left to have a headache or stomach ache or both for the rest of the day. it happens anywhere from 1 to 7 times a day. the length of the spell and the intensity of the spell varies as well. it happens for about a week and then goes away for a few weeks and then comes back again. what could be wrong?