you haven't mastered a 2 wheeler yet
it's a combination of the muscles rebuilding themselves and the lactic acid.
Exercise breaks down the outer la<x>yer of the muscles causing soreness. during healing, new tissue is added. this is what causes muscles to grow larger and stronger, or sometimes just stronger, depending on the density of the new scar tissue.
Once you get used to riding a bike every day for a week you want feel sore. You feel bruised because you aren't used to riding a bike yet.