What Causes Severe Anal Pain While Passing Stools During Periods?

For the past several months when I am on my period I get severe anal pain when I am having a bowel movement. This only happens when I am on my period and typically the second day (maybe third) when my flow is the heaviest. Is this something I should be concerned about or is it just a cause of menstruating? I am 35-years-old and have normal periods every 28 days.