Here's a riddle: What goes "pop" and shouldn't feel like that?
Answer: My Achilles tendon!
Last night, not even halfway into my first Ultimate Frisbee game of the year (and of ever, for that matter), while starting to run, I hear a loud (to me) pop and a feeling like something hitting me in the back of my foot/leg. I look back, but nothing's there. What is there is a growing pain and an inability to walk. I had to be helped off the field, got it wrapped up and iced and lay there trying not to be too uncomfortable. I went into the walk-in clinic (or should I say hobble-in clinic) today and luckily it doesn't look like my tendon is actually separated. Just a nasty sprain. Anti-inflammatories and ice packs for all!
I had even gotten back into doing 5k a few times a week, but that's on hold for now.
Moral of the story: Stuff can happen when you get off the couch.