I do the same exercise as you and I always feel better if I just slow it down a couple of days and make it a regular or brisk walk. Also, are you eating enough? If I have too much pasta and bread for too many days in a row, my weight loss slows and I begin to feel sluggish as well. I just replace a meal with an omelet, usually dinner. That helps. Maybe you haven't been drinking enough water? Anyways, I hope you feel better and get back to your runner's high!