Hi Tushar, blood in stool could be due to various reasons. However exact site of bleeding should be identified in order to treat the condition. In your case most probable cause could be bleeding piles as you are working out in a gym that raises pressure inside your abdomen which can lead to bleeding piles. So I suggest you to first visit a anorectal surgeon or a general surgeon who can easily diagnose this condition. Black colour stool also known as melena is seen commonly in upper GI bleed commonest site being a ulcer in stomach or duodenum, but that is usually associated with burning pain in abdomen. So if no anorectal bleeding site is found you should go for upper GI endoscopy which is relatively invasive but completely safe procedure now a days. if you are having frequent passage of blood mixed stool with abdominal cramps and pain around anus it may be a case of dysentery which is very common in India and antibiotics are all that you require