How long before this are you eating? Do you eat something at the same time every day?
I'm wondering if you're having a blood sugar crash. Try eating a few saltine crackers, or have 1/2 cup of fruit juice.
The other thing that might be happening is you're having what's called a silent migraine. A silent migraine is where you get a lot of the other symptoms of a migraine except the head pain. When I get them, I feel so cold, my teeth chatter.
Look for patterns during your day to see if you can figure it out. Maybe it's something you're doing, or eating.
If none of these fit, or you can't find the pattern, you should see your doctor about it.