I'm 23 too OP and recently went to the doctors with the same problem in maech after being quite poorly. She was quite dismissive, but she said it's so unlikely but could still do tests of I wish, but because it was only over the course of me being poorly and on meds it was unlikely to be cancer.
I decided to wait and see, as my anxiety used to make me make quick rash decisions which was never good.
I stopped googling as I would always convince myself I was dying thanks to Google.
When i got over my infection my body and bowls went back to normal and no more bleeding or mucus.
I occasionally get bleeding now.. I'm pregnant so the constipation is making me bleed a bit.
IBS can result in bleeding, as any change in bowel movements, hard or soft, can cause piles which are prone to bleeding anyway.
I could be wrong on this one, but I'm pretty sure red fresh blood is usually a tear or pile ect, and dark red/black blood that resemble coffee grounds are a sign of a more serious condition, such as an internal bleed.
Cancer is very unlikely. I dont think your doctor wants to be dismissive but the NHS just doesnt have the funding to be teating you left right and centre for something they're professionally not concerned about.
If your symptoms persist for more than 1 weeks start to finish, let your doctor know. Although if it is related to a condition like IBS then it's likely to be a long term thing for you and you may have better and worse days.
I know it's hard worh anxiety, but try not to dweel on it. I can almost 100% Guarantee that if you preoccupie yourself for a few days and try to convince yourself there is nothing wrong, its just a change is appetite and diet, your symptoms will go.
It is proven in people like us that have health anxiety that we can imagine symptoms, we can literally think ourselves poorly, we can be so convinced we have a disease that we can actually feel the symptoms of it.
Try not to worry :)