It could be that on some days you have a combination of things going on.

For instance, nutrition. If you go out on an early morning ride, and the night before you ate a lot of greasy food, then you're not going to feel that good.

Or it could be you're on medication and you might have forgotten it and are feeling the results of it.

Or it could be that you did not get enough rest and your body is telling you that you are still tired.

Or it could be that you have Type II Diabetes and don't even know it.