This is just part of the overloading process that leads to the adaptation phase after which your base level of fitness should hopefully have gone up a notch.
What you do need to do though is listen to your body and keep an eye out for signs of over-training. This is best done in the morning. As Joe Friel puts it in his Cyclists Training Bible - you have to listen to your 'Morning Warnings'. ''Every morning on waking your body 'whispers' what it can handle that day. The problem is, most of us refuse to listen''.
1. Sleep Quality
3. Stress Level
4. Muscle Soreness
5. Heart Rate
For the first 4 rate yourself honestly from 1 to 7 with 7 being the worst. Also record your resting heart rate whilst still lying quietly in bed. Compare this to your known normal resting heart rate (your average morning heart rate taken during your recovery week)
If you score 5, 6 or 7 for the first 4 indicators or your heart rate is +/- 5 beats from your normal resting heart rate then consider it a warning that something is wrong.
A couple of 5's might simply mean you need to reduce the intensity or distance today but a couple of 6's probably means that you need a complete day of rest.
You are best not ignoring these warnings as eventually your body will become fed up with you not listening and give you a cold or an injury instead, which is going to result in a lot more than 1 day off in the long term!
If you have training diary then enter these morning tests on it. You might find that you have a pattern forming that you can preempt in advance or discuss with your coach.