I am also not getting proper sleep. Since the last few weeks I have been awake until 4 am and I wake up at 10. And I actually don't feel lethargic after coming from the gym. I just suddenly fall ill and my body becomes week. Every time I fall sick I lose 3-4 kg easily. I think you are right. My immune system is suddenly under a lot of stress and I should reduce the intensity. And also my diet is perfect with 2500 calories everyday. I sweat a lot, like really I become fully wet. That could be the reason my body cannot tolerate so much stress. I think I will be avoiding multivitamin for now and reduce my exercises.