Soreness from exercising is not rare and not necessarily a concern. The most common cause of muscle soreness is from microscopic breakdowns in the muscle- this can lead to Delayed onset Muscle Soreness AKA DOMS. DOMS comes on 24-72 hours post activity. This is why you may feel great the day after but not the next day. Is it okay to exercise when you are sore? It depends on how sore etc. You should never exercise if you are in PAIN, feverish, or sick. Find out more here!