Have you ever tried out a weight training exercise you haven't done before -- so you pick a weight to start... One rep, you think, "Oh this is easy. I may need a heavier weight." After three reps, you start feeling it. And after six reps, you literally cannot do another one?
Is this normal (especially for underworked muscles)? Or could it be a sign of too much caloric deficit or something?
I haven't had that particular experience but it's muscle fatigue, not caloric deficit.https://en.wikipedia.org/wiki/Muscle_fatigue