Nuclear stress test sudden dehydration with water

I had a nuclear stress test last week during which I had some kind of reaction. I got an extreme sick headache with nausea and dizziness and my blood pressure was something over 96 2 hrs later. The stress test showed no problem with my heart, but my pressures zoomed to over 200 even though I took my BP med. I had been drinking water and apple juice that morning, but they told me I was dehydrated. I started feeling better after I drank multiple electrolyte drinks. How do you get dehydrated while drinking water and juice? Is this a kidney thing? My father has many kidney problems. I am desperate to avoid that feeling again. I had it all day and am still having a slight headache 3 days later when I don't drink electrolytes. Does anybody have a clue here? I may never have a stress test again!