It's important that your doctor understand things in your body that could be warning signs of something bigger. Most doctors are not going to run a battery of tests on you every time you come in. Unless you let them know something's changed within your body, they have no way of knowing they may want to look deeper into your current health. And while you're at the doctor's office, you may want to talk about your colon cleanse possibilities.
No one is saying that you should just live with severe constipation. But you do need to let your medical practitioner know that you are suffering with a problem that could be something more than just irregular bowel movements. If you also have other medical conditions or medical concerns, it is wise to talk to your doctor about any colon cleanse you are considering taking.