How come i start shaking uncontrollably when i have a panic attack?

That's part of it.: Uncontrollable shaking may be one of the symptoms of the panic attack. People sometimes mistake panic attacks for heart attacks and other serious medical disease. You body gets all off track from the impact of the "emergency" chemicals/hormones that are released and the imbalanced o2/co2 ratios from rapid, shallow breathing. It feels awful, but makes sense. It's good that you can get help.

Adrenaline: When people have a panic attack they feel threatened (often by the fear of dying, losing control or going crazy)...When we feel threatened, our adrenal glands pump adrenaline into our blood stream....The high levels of adrenaline impact many parts of our bodies including our muscles. It makes our hearts race and our hands shake. There are medications that can help. Therapy is best.

Fear vs Excitement?: The physical manifestations of anxiety/panic r the direct result of the cascade of physiological events that get triggered when our primitive fear & rage brain centers sense danger -- or a chance to mate! these very same centers are highly active during sexual foreplay & activity. Think about it: is ur "uncontrollable" shaking so different than what happens when u climax (reach orgasm)?