You are very young for cancer and your symptoms sound more like IBS (Irritable Bowel Syndrome). This causes alternating diarrhea with constipation, stomach pain and the feeling that you need to "go" but can't. With the straining you have done, it's hard to believe you don't have hemorrhoids, I wouldn't rule this out just yet. It's always best to see a Gastroenterologist when you have any concerns about intestinal problems. It also sounds like a fissue which could have occurred when you had the painful BM and your GP just can't see it. The fact that you are young and have no family history of colon cancer says a lot in your favor, I truly think you don't need to worry about cancer. I have an heridiatry colon cancer in my family, and even with this you don't see colon cancer this young. I wouldn't mislead you, so try to relax knowing the gastro doc will be able to give you definite answers, and not one involving cancer. I will be thinking of you and wish you all the best!
The rectal bleeding and anal burning you described is highly suggestive of an acute anal fissure. The amount of bleeding is actually more impressive than the actual volume. The complex of alternating diarrhea and constipation associated with abdominal cramps is potentially compatible with irritable bowel syndrome. Generally a bulk fiber laxative such as . Metamucil is prescribed and topical Nifedipine applied to the anal area may be helpful. Statistically, the risks of having colon or rectal cancer in someone as young as you is very low but probably should be investigated by colonoscopy. FOBT does not have any diagnostic value in your case. Ask for a referral for a colonoscopy.
I was in your situation few months back, googling a lot on colon cancer and convinced it to be there. But with consultation with GI and tests and examinations turned out to be a poly which was removed two days back.