Fatigue is cumulative across all activities. Having a bad day at work can negatively impact your ride. Your kids or spouse driving you nuts can have an impact. Not getting enough sleep can hurt your ride. Sometimes you just have a bad day. For shorter rides like the ones you mention, I doubt nutrition on or off the bike is a major factor unless you did something totally different like just starting a diet or choosing to eat spicy Indian food for the first time.

Also, you don't actually get stronger and better during your training, it's proper recovery from good training that leads to improvements.

In the case of truly feeling like dead legs, I would either take a day off or do something very easy because the body is saying it needs more recovery. Active recovery could be a leisurely walk, easy swim, or a ride where you keep your heart rate in a low recovery zone (or if you don't have a monitor, ride only as fast as will allow you to sing the entire ride without ever getting out of breath). It's too easy to get in a trap where you want to ride "fast" everyday which in the end can be counterproductive to your fitness.