Doctors giving up, told it's nothing?

As mentioned before have you had a Colonoscopy? There is one disease you haven't mentioned is Chrones Disease. Very difficult to diagnose. They might be doing a Pet Scan. A Pet Scan mainly focuses on finding Tumors in the body. Most tumors absorb sugars very rapidly. You are given an injection and then they scan the body. A tumor can be picked up because the tumor will turn red because the high sugar absorption will heat up the blood flow. Good Luck.