Could this be colon cancer? Or IBS?

I am a 21 year old female with no family history of cancer. My mom has celiac disease and her mom (my grandma) has severe diverticulitis. My stomach issues began in November 2018 (my freshman year of college). The issues went away for a while but came back recently. I have frequent episodes of alternating constipation and diarrhea (sometimes in the same day), I've had small amounts of bright red blood on the toilet paper (but not recently), lots of lower abdominal cramping (almost always the lower left quadrant), and feeling of incomplete bowel movements. I am also extremely gassy and pass gas literally all day (sometimes up to 70 or 80 times per day) and I have cramps after bowel movements. I went to my GP and she ordered a CBC/Chem bloodwork (normal), abdominal X-ray (normal), fecal lactoferrin (negative), and fecal occult blood test (negative). I have struggled with health anxiety and hypochondria for years and I am absolutely convinced and terrified that I have colon cancer. My GP assured me that I likely do not have cancer and this is very likely IBS brought about by stress from college and my diet considering my family history and symptoms. I have also not lost any weight since my symptoms began in 2018 and I am not anemic. My main concern is the lower left abdominal cramping/pressure/pain. I am convinced that it is a tumor. If it were cancer, wouldn't they probably have found it by now? I am so concerned and I am convinced that it is cancer. Thank you in advance!