Owwie? But how?

I'm up and about, feeling really good this morning from being all drugged up from cold medicine. My task today is to buy some groceries. As I walk down the stairs outside to the car, I realize my knee hurts. I ignore it. But as the day progresses the pain is more and more noticeable. When I return home, I head to the bathroom to check out my knee. I roll up my pant leg to see what's wrong. I notice that there is a slight scratch mark and a huge black bruise. What the heck?! How did that get there? I thought long and hard but I couldn't recall hitting my knee on anything these past couple of days. Dear god, what if I have that weird disease where you get bruises all over?? Eeks!! But wait...I remember now. I fell on my knees on NYE! ahhahahahahaha! It took a whole week for the bruise to develop and for me to feel pain. Is this what happens when you get old?