Massive drop in run timings?

Just a quick question. Has any of you experienced a big drop in run timings for no apparent reason at all? I had a military fitness test consisting of the 2.4km/1.5 miles run. Prior to that, I have been running in the 12-13min range at a stadium track twice a week.

During the test, after my first 2 laps at a similar pace to my training (1:50-1:55), I felt flat and my next laps withered to almost 3 minutes per 400m, resulting in a terrible score. Can't wrap my head round it though.