+1 for electrolytes. Hydration is obviously the key but too much water will only make it worse. We are the lucky few who don't have to worry about sodium intake. My BP runs 80-90's/40-50's. Freaks my co-workers out. I have to increase my sodium intake in the summer and before long rides. Orthostatic hypotension is a daily thing for me. I've learned to switch positions slowly and I cannot ride if it is near 90 or above because then I just get dizzy no matter what my position. There are medications you can take. Your doctor may want to do a "tilt-table" test to determine if you need them.