I have intense pain at anus during defecation during periods. is there any known cause / pathology?

Endometriosis?: Ask your gynecologist whether this is a possibility and if so, whether it could be treated.

Agree: The association with your period makes me immediately consider endometriosis and the anal pain may be referred pain from elsewhere near the anus. Your ob/gyn should be able to assist you, but may also want to send you to a gastroenterologist, depending on the specifics of your case.