I can understand some muscle discomfort the day after cycling-but not discomfort that goes on for 2 or 3 days. If the doctor hasn't suggested any compressed discs in there-I am wondering if the facet joints aren't arcing up on you. You are so active-with the running and swimming-that big muscle groups are nice and limber-so-yes-I would wonder about posture. Can you get a check done on that?