How did this happen?

Once upon a time, I could go outside, play all day, and the only physical thing I would feel at the end of the day was tiredness. I would wake up the next morning and do it all over again. What happened to that person? Where did she go? How come now, after a day of swimming, of swinging on a trapeze into the water, of diving off a high dive, does my body feel like it's been picked up and thrown in front of a semi-truck? From swimming??? The joys of growing up and becoming an adult.