I did start exercising a few months ago out of worry. I'd do an hour and a half of cardio a day, split between two sessions of 45 minutes with a pulse of 150 to 180 (i'm 19). I also did some weights. Although over time it seemed to worsen my symptoms. I stopped this routine and my symptoms subsided more but are still there.
I'm getting a 2 week holter monitor in october also.
Don't know what may cause it, but if you have a history of 70 at rest, then 50, even 60 could cause some dizziness, headache and some of you other symptoms.
Irregular heart beats is quite common and usually benign, I believe and it seems so do you doctors. It is good you have been checked, by doctors including a cardiologist. Some of you symptoms suggest a minor heart attack, if there is such a thing.
As for diabetes, I think not if not in your blood test. But watch you sugar, ti can cause lots of problems, you may want to try cutting sugar out of you diet and then focus on dropping a few pounds, you will be surprised how much it helps.
I am not obese, at 6' 5" and 230 pounds, but I was as high as 250 and I can tell the difference, and benefits of losing abouit 10% of my weight, closer to 8%. A little loss can help big time.
I should also add that my resting pulse has changed significantly. My usual resting pulse used to be around 70. Now, it seems to float between 50 and 90 at any given time.