I was exercising frequently but slowed down in March / April of this year. I had thyroid cancer and my meds were affecting me.   In May, I started working out again but only did 1/4 of my normal workout. I used to walk 3 - 5 miles a day and on M, W, and F I would do about 45 minutes on the bike using the program Hill on level 6. I would go between 95 and 110 rpm. No big deal. But when I started back up again, I did the Hill level 5 and around 75 rpm. About 10 - 15 minutes into it, I got dizzy, lightheaded, and an intense pressure went up my head to give me a 8 hour long headache. Every single time I try to exercise the same exact thing happens. No matter what type of exercise I do. I had 3 abnormal EKG's but then they gave me an ultrasound on my heart, a stress test with radioactive material put in via IV, and a 24 hour heart monitor. They said the tests came back normal. Now I'm worried and have no idea what could be wrong with me and the doctors are blowing me off.

Optional Information:
Female , Age: 29

Already Tried:
EKG, ECG, Stress test with radioactive isotope, 24 hour heart monitor