Excuse me if this is discussed often. I searched several times with different combinations of the most obvious search terms, and didn't find anything.

So, my question is about dead legs. Yesterday while riding my speed felt pretty normal. I have a hill that I have to climb out of my neighborhood, and was getting 15-16 mph up the hill. That's about average for me. On the flat, or slight incline, after the hill I was averaging about 18-20, also average. I got about 20 commuting miles in yesterday. I did not ride my rollers yesterday, so my workload was less than usual, almost a rest day. Today I put in some time on the rollers, and was struggling to muster any effort to push it above 18mph. I usually spin about 23mph (ride fixed, no option to change gearing so this is around 100-110rpm) for an hour and a half. When I left the neighborhood, I was just so listless and slow. About 12 on the hill and 16 on the flats. I didn't feel specifically tired, just felt like I had no energy. I can't blame it on change in diet or anything. This happens to me occasionally, and I want to know why. I also want to know if there is something I can do to avoid it, or work through it, or even plan for it on a day when I can afford to not ride. Thanks.

Edit: I had the wind at my back today, and yesterday it was calm. I should have been faster today.