I find generally these days are actually some of the best in terms of practice productivity, and generally come before some of my best practice days. I think it's either that you are not actually playing any worse, but realising mistakes you were making that you did not notice before, or that your brain actually working out some new neural pathways that are going to be useful in the long term... Or possibly both.
For instance, I have noticed that when I'm learning a tune (no matter the instrument), I actually need to learn it at least 2 or 3 times over several weeks. The first time, it's very much a conscient approach (this finger need to go there!). Then after leaving it rest for a while, I'll have forgotten some of the tricky bits, which I need to relearn until they become more natural.