[color:"orange"]As I said I cannot recall the EXACT moment when I begin to lose my stamina[/color]

I'd guess since the first level of the abbey, in the north east corner. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />
There is a Halberd of the Rock in the abbey with a description like 'there is something strange about this weapon', which will drain your stamina. Check your weapon inventory, and drop or sell it.