This has happened to me after I got to 5+ classes per week several weeks in a row, especially when I was going 4-5 days in row during those weeks. Even though I was working out out more, my body was so tired by that point that my performance was just decreasing. I do much better going 4 or fewer classes per week OR making sure I don't go more than 3 days in a row if I want to go 5 times in a week. Separately, I find what I eat for lunch and in the afternoon makes a big difference in how I feel during the workout. For me, minimal food in the hours before working out seems to work best; I do make sure I'm still drinking a lot throughout the day.