Every time I get my period, I have a bowel movement as well. Why is that?

Not necessarily bad. It is hard to know what you mean. A period lasts for a number of days, so are you saying that you have a bowel movement the entire time? Yes that would be abnormal and you should seek medical attention. The other possibilities are not so bad. Perhaps you're saying you have a bowel movement when you use the bathroom every time during your period. Could be that you valsalva (bear down).