I am in normal heath as well, I even get easily fatigue after some running etc.
In my first shamanic breathwork tryout , I could barely reach 15 mins and then stopped. Because I had got already some interesting emotional "visions" by the 8th min or so & then i got distracted by my own crying and stopped.
I repeated sessions for 5-6 times at least later on for the past few months- but i never reached the "visions" again, although I could reach 20 mins these times. In each time I had numb hands with crooked fingers , numb nosetip & lips.
I guess surrendering yourself to the situation by lightnening your breaths would work ?