I would think it's very unlikely to be cancer. From what you say, it sounds like your doctor is hardly even considering that possibility, that she is just checking to see whether it's hemorrhoids or IBD.

About a year and a half ago, I had rectal bleeding nearly daily for weeks on end, as well as weight loss (about a stone in a year), trapped wind and occasional loose stools (maybe once a week). Like you, the doctor could feel nothing abnormal, which was what worried me. I really only went to the doctor to get anything serious ruled out, but it was scary when they didn't.

I was referred for a colonoscopy as my grandfather may have died of bowel cancer and the doctor was young and was possibly being a bit over-cautious. The nurse in the hospital said that it is very common for piles to be missed during rectal exams, if they are internal and that it was still most likely piles and the consultant said that if my mother reported the same symptoms I was, he would consider cancer as a possibility, but as I was only 40 at the time, it was the least likely option and I would be the "unluckiest person in the world" if that turned out to be the explanation. He said the odds were less than 1%.

At 25 and with no family history, I would say you are at even lower risk than I was.

Of course I am no expert, but based on my own experiences, I would say there are many more likely options than cancer - hemorrhoids, a fissure, ulcerative colitis, Crohn's, a polyp, certain infections...