I doubt it's the exercises or workout. It's likely stress, lack of sleep, not eating enough, lack of water or undereating one of the macronutrients.
You're a teen. You should feel vibrant and healthy. My guess is that it's food related. But again, it's a guess.
STRESS, yeah, that could very well be it.
Just started college this week, was very stressed.
I feel vibrant and healthy, its only when I was lifting that I felt weak, not in everyday life.