I had a bad workout / I was weak today. What happened?

There are such a wide range of possible answers to this question that trying to diagnose it is really not worth your time until it becomes a consistent problem. Maybe you didn’t sleep enough. Maybe you slept too much. Maybe you ate too much too close to your workout. Maybe you didn’t eat enough. Maybe you were dehydrated.  And so on. There’s really no way to know for sure for one-off bad days.

Bad days happen to everybody, from absolute beginners to the most elite, top 1% people. The best thing to do is just not worry about it, stick to your routine, and get back to the gym for your next scheduled workout like it didn’t happen. If you are consistently over a period of weeks or more having problems, that’s the time to start looking for a problem.