I've had similar physical reactions. For me, it wasn't usually dissociation; I was usually fully present (although I can think of one exception to that). It was more an anxiety reaction.

I also had a habit of not breathing. I would hold my breath and not realize I was doing it. My therapists tuned into that pretty quickly and began actively making me aware of my own breathing (or lack thereof) because there were times it resulted in hyperventilation, dizziness, severe physical tension, headaches, etc.