Blacking out and dizziness can be caused by a too low blood pressure or heart rate, or both. You say you are on BP medication, what is you resting BP? If it is real low, not likely given you normally have high BP, the medication may be too strong a dose or type. BP medication may also lower you pulse.
I have had problems in the past adjusting to high dose beta blocker, e.g., 100 mg of Metoprolol twice a day. In the beginning by BP would go down in the range of 80/65, best I can recall.