listen to your body. maybe instead of lifting on those kinda days suggest other activities, like going for a long walk or hike, tai chi, or just relaxing. like van said, sometimes just getting the blood and lymph flowing with other activities first will stimulate energy, but still respect the body and don't push yourself to do more then you really feel like. plus its spring, the body notices the change in light and temp and is more prone to random energy fluctuations. don't over think it!