Sometimes after teaching a lot of yoga (5 hours today) I actually end up feeling tighter. In my last class today I could hardly touch my toes in a forward bend. In the morning I had my head on my knees in the same pose.
One of the most important Buddhist concepts is that of impermanence (anicca). Nothing in the entire universe stays the same. Therefore, getting hung up on something staying a certain way is a recipe for frustration. This applies to little things, like getting upset when it starts to rain on your picnic, or big things, like when someone close to you dies. Of course we all intellectually know that nothing is forever, but we don't really believe it in our bones. I think, in all of us, there is a little voice that whispers, "Sure, everyone else will have to deal with change, but somehow we're going to beat the system."
Impermanence is especially true of the human body. I heard a report about how within 7 years every single atom in our body is replaced save for a few brain cells which we have for life. So, if you look at your hand right now, there isn't a single atom in there which was with you in 2001.
But of course sometimes we feel that our bodies have changed just over the course of 7 hours, as I did today in the afternoon forward bend. These rapid changes in my flexibility levels used to take me by surprise, but now I just roll with them, without trying to force the pose back to where it was last week or where I think it should be in my image of it.
And it's interesting to apply these ideas to the PCP. Every ounce of muscle I have put on will soon fade away, either in the near future if I stop training or in the not so near future as I get older and just can't maintain it anymore. It's all destined to weaken and die. This is a good thought to have. It quickly deflates any pride I might develop in my progress, and keeps everything light.
Many people ask, "how can you think like that and still be motivated?" For me, impermanence is a great motivator, because I only have today to make something of the opportunities presented, as tomorrow everything will be different. And I'm strong right now, today, which is made all the more pleasant with the knowledge that it will all fade away before I know it.