What Causes Dizzy Spells And Rapid Heart Rate?

I am a 64 year-old-woman in good health, my pulse, however, runs in the 80s. A few months ago I started exercising on a stair treader. During exercise my bp will go up over 150bpm and my bp will go down to around 90/53 and I begin feeling nauseous. Naturally, I slow down and things even back out again. After exercising, my bp is usually in the normal range, my pulse, usually around 115-125. However, as I wait for my pulse to get back down below 100 I notice that my bp has fallen rapidly and is back around 90/53. It takes another few minutes for my bp to go back up. My pulse is usually around 93 at that point. I have had problems with dizzy spells in the past and have seen a caridologist. My stress ekg and others tests have always come back normal and my cardiologist feels my dizziness if caused by another unknown issue. Any suggestions?