I have a similar fev1 at 25% and am currently being evaluated for transplant. When I did my walk test I desated to 83 after about 400ft or so. I can't exercise without O2 now and this wasn't the case a couple of months ago. It seemed for me like it changed overnight, one day I could walk on the treadmill for 30+ mins with no problems and now I have to be on 6L of oxygen to do that. Some days are better than others, maybe you were just having an off day?