I've noticed that sometimes when I go in to lift, I have an awesome lift; i'm able to do more sets and more weight, and the day after I can really feel it. Other days, like today, I go in to the gym and I'm just not happy with what I did. It seems like my muscles get fatiged and are maxed out a lot quicker. Is this just how weight lifting goes, or does this depend on your diet? Can anyone help here?