Painful bowel movement

okay, i don't have hemorrhoids or anything, but when i poop, it really hurts. not my anus, but more like up in my rectum. i dont' strain or anything, because i don't want hemorrhoids, so i just kind of breathe through the pain. it will get real intense and then go away, and so i push a little, you know, to get the poo out. what makes it hurt so bad? is it just the weight of the baby on my colon? or might there be something wrong? please help, i;'m so embarrassed!