My abs don't hurt but I can only do 12 sit-ups, I've never had troubles before what happened?

I've been in the military for 4 years and am required to do sit-ups as part of a fitness test. I recently had a fitness test and only was able to do 12 situps before i couldn't do any more, at all. That was in the morning, over 20 hrs later I tried to do some more to see if it was a fluke, but that time I could only do 2. My abs don't hurt at all, but feel very fatigued when I try to start to do sit-ups. What the hell is wrong with them. Every test i've taken I've ped with flying colors up to this point. Even when I've been injured or lazy for a month or more, I still produced over 53 sit-ups in the allotted 2 mins, which is the minimum. I need to figure out whats wrong otherwise I could get demoted. My hip flexers seem fine and my lower abs seem to be able to put forth some effort. What sort of injury could it be if everything seems normal and doesn't hurt, but I can't even perform at 20%?