Thanks for the replies.
Yes, it's not a muscle because of the sharp pains. This morning, about 5 days after the pain started, was the worse pain yet. This doesn't feel like routine soreness from working out, it feels more like a broken bone or something! But I'd know if I broke a bone, right? And the pain wouldn't get worse when I woke up. It's strange.
I'll see a doctor if it's still there in a week or so. In the meantime, no more long walks for me.
I just love my walks so much!