Originally Posted by Kheapathic
Well considering there are two things at that part of the game; the first one is you're in the Naked Camo which causes your stamina to drop quickly, or there's...
There's also the chance Snake has Leeches on him. To remove them you'll need to use the cigar in the healing menu. There's also the chance you're wearing the Fear's camo which helps your camo index but will also drain your stamina rapidly.
ffs I have been wearing the fears camo since I got it and my stamina drops dramatically. I thought it was because I have 2 arrows in my arm and 1 in my chest. thanks for sorting that out.