Delayed onset muscle soreness (DOMS) is the result of muscle fiber injury from unaccustomed exercise. This experience of soreness is not uncommon, particularly if you are just embarking on an exercise program or changing the normal exercise routine.
This soreness may be felt 12-48 hours following the exercise, and is often accompanied by muscle stiffness, fatigue and weakness. This, however, is a normal response to unusual high intensity exertion as your muscles adapt to the new stress. Rest assured, the pain and discomfort is worst in the beginning and will start to subside generally within 2-3 days.