Hi dizzy128 and welcome!
I am sorry that you are currently dealing with this issue.:( I know that it is scary and confusing to go through.
What you are likely experiencing is the sensation of a near faint or seizure type episode. This is a pretty common concern with those suffering anxiety.
It is rarely actually occurs...but the feelings/fears of it happening can be very convincing!
The good news is that you have checked out well from a physical standpoint, so you can now focus on reducing the anxiety and symptoms.;)
Here are some exercises that should help. Practicing these relaxation skills and breathing techniques will go a long way in reducing your symptoms.
Self Help sites with breathing/relaxation exercises instructions
Panic attack relief exercises:
http://www.healthcentral.com/anxiety/c/73076/34334/breathing-lessons/
http://www.healthyplace.com/
http://www.stress-relief-exercises.com/
http://www.ehow.com/video_4405003_breathing-exercises-panic-attacks.html
http://www.psybersquare.com/anxiety/panic_relax_I
Commit to trying these 3xs a day for at least 10 min a session for a week and see how you feel.
Keep posting with us as well. Support and understanding is key. We are here to listen anytime.
Scaredy Cat