If you have had a thorough exam, and medically cleared, then it sounds as if it is anxiety. There are many behavioral management and medications to assist with this.
The trick with any treatment is consistency. I don't know if you have tried managemtn techniques before, but these are usually easy to do, and work pretty well.
A couple of things to try. Practice deep breathing exercises first to relax your body as much as possible. Lie down with your spine straight and no limbs crossed to allow adequate blood flow. Breath in through the nose and out through the mouth. When inhaling, allow your stomache to come out. When exhaling, stomache should go in. this helps you force lair to the lower lung area for deeper breathes. Practice this for at least 10 minutes each day.
Once you have mastered the breathing, add a few things. Close your eyes and visualize the air you breath in as a color you see that would be calming, example: blue. Feel and see the calming color move into your body. When exhaling choose a color to represent the anxiety/stressor and see/feel it moving out of your body.
This is just one example of a behavioral mangement. Any technique must be practiced daily, and when you feel stress, regardless of where you are, your brain is trained to use some calming behaviors like this. There are many books with techniques to manage this.
Not sure if you are on anti-anxiety medications, but this can help also.
You are not crazy. You just need to learn to manage the anxiety a little better.
Please let me know if you have further quesitons. Gina