Cardio question

hendrijm

I have noticed lately that when I do cardio I feel more energized the day of, but exhausted the next day. I don't feel like I'm doing too much when I do it, I have my heart rate in the right range, it isn't so hard to do that I must stop in the middle of my 30 minute video and collapse. I am unsure how to reduce it while still getting in the right range, or whether doing too much is not the problem but something else. I do have adrenal and thyroid conditions. I have not noticed this with weight training, only cardio. Has anyone else experienced this or have any suggestions? The workout DVDs for shorter times are more intense than the 30 minute one and I think that my heart rate might be too high with those.