Why do i always have sharp belly pains when constipated????????/?

Im a 29 yr old female that is always constipated and when i need to poop i get extremely sharp pains. The pains are so sharp that I feel like im going to p out. The pain comes and goes until i have a bowel movement. Once I start to have a bowel movement it starts off with little pebble like stool and then it becomes a little liquidy and then a huge hard like stool will come out followed by diarhea. Sorry i dont mean to sound so detailed just trying to make u guys see what im talking about. After i have a bowel movement i still feel pain but its like a sore pain and it usually goes away after a while sometimes a day or so. I worry i might have a tumor or cancer or something seriously wrong. Any ideas on what might be wrong??????????