Overtraining? Underrecovering? Undereating?.. resting? Take your pic. It's a delicate balance of everything combined, with effort in the gym, which takes us to our goals.
I used to "undereat" many years ago and never understood why I was always ripped but never got "big muscles". NOW I know it was undereating/overtraining.
You need the perfect mix.
Test your heart rate upon waking in the morning. If it's higher than duriong the day at rest, that is one sign of overtraining. I think that's how I remember it.
Also, if you THINK you're on the verge of overtraining, than a week off from the gym will do ZERO harm, but could help tremendously.