You are probably low on iron, try eating more red meat in your diet.
It could be low blood pressure. If you see a doctor make sure that the person taking the blood pressure waits until the cuff registers the lowest level. Often they stop when it enters the normal range. I am very aware of this problem as I was given a prescription that should never be given to someone with low blood pressure and mine dropped to 80/40 because of it. Very dangerous. All because of a lazy nurse. I have now observed the same practice in two other offices.