sore?

I woke-up this morning with a rather unusual feeling - soreness! Sometimes during training you push through a couple hard weeks, but wonder if you're doing enough as while the body is tired, it ceases to get sore... but not today. The tops of my hamstrings are as tight as a couple of piano strings. Which in some ways makes me feel like I got what i needed to out of last week, but I'm also curious why this soreness would come-on now after a day of rest, then a hrm zone 4, 2hr ride yesterday - which is nothing out of the ordinary. Ahhh, the mysteries of training...