worried

 am a 24 year old female with no family history of colon cancer. I have been experiencing some very disturbing symptoms. It all started in early September when i noticed i had blood outside and inside of my stools. they were brown and formed normally, but i saw the bright red blood. Then i started having lower left pelvic pain. I was very concerned so my dr ordered me a sigmoidoscopy which came back normal but showed hemmoriods. I have had blood work, urinalysis, a ct scan of my abdomen and a pap smear which all came back normal. i also have had an ultrasound of my lower pelvis which all showed normal organs with no cysts or abnormalities. Now i am having lower back pain, also a clear discharge which looks like there are bubbles in it, and sometime it is yellow with no bubbles just really slimy. my stools are thinner and smaller most of the time now too. the stool isn't too hard or too soft. i am so scared i have colon cancer, or some kind of intestinal cancer. i have been trying to contact my dr for further tests but they seem to think it is nothing. they are very quick to mention my age and how i have no family history of colon cancer, and how unlikely it is that i have cancer.  i know there is something wrong and will continue to be persistent with them, but in the mean time i would really appreciate any advice or symptoms anyone else has had and the outcome. thank you in advance. Courtney