I have those days!
It could be because your body is tired or something you ate- I don't really know. I do know that sometimes I will have a day where to put in 20 minutes of cardio is torture. Other days I can do 60 minutes with energy to spare.
I actually had one of those days last night. I was very alert and felt fine, but I really struggled on the cardio. I think I was just having a "jiggly" day. Not only was my body tired, but I felt very "jiggly"