carrellh - you most likely stalled... it's really hard to judge what kind of speed you need to be going to maintain lift and flight control when you're going downwind. it may look like it's going fast enough but relative airspeed my be far below the stall speed. when i'm dead sticking from a high altitude down wind situation, i always make sure the down wind leg and turn are down with a slightly nose down attitude. this ensures that i am not going to stall. then only when i line up for approach against the wind and you feel that i have enough speed, then i ease the nose up.

i saw two plane literally fall out of the sky last weekend due to the winds. they all tried to turn in the wind with too little speed and ended up losing flight control. luckily both are easily repairable.