In other words, your blood is too thick. I had the same problem. Your brain isn't getting enough oxygen and your body is trying to make up for it. Don't know if you can take aspirin or not, but that does the trick. ... But then again, that may have been your Dr intention when he put you on the vitamins. Better check with him.
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Psalm 119:105 Thy word is a lamp unto my feet, and a light unto my path.