Hello

I know exactly how you feel. I had a similar experience and ended up having a colonoscopy and they found a big polyp which they removed in December - I am 24 female btw. 
 

I had a qfit test from the GP (you usually only have these over 50 or 55 I believe) which I would push for - a normal stool sample doesn't always pick up microscopic / mixed in bleeding. Even though mine came back super elevated, I was told I wouldn't be seen on the NHS for up to a year due to my age. Luckily I was able to get a private referral through my work health insurance and I'm so glad I did as my symptoms (bleeding & pain) have now gone and no risk of the polyp turning into cancer. My gastroenterologist told me it's super rare to have this in your 20s so will need to have yearly colonoscopies to make sure no polyps grow back. 

also to mention I had some really obvious bright bleeding for years which freaked me out but this was actually caused by hemmorhoids (I still have this occasionally!)and upset stomach (IBS which I still have :neutral:) so the really alarming bleeding I could see & symptoms were harmless. The polyp bleeding was small and mixed in with stool which has now stopped and I had some cramps and more severe pain which has now stopped after my procedure. 

it's more likely to be hemmorhoids & IBS or maybe colitis but please push and make sure you find out, don't let them tell you this without testing you properly if for nothing else peace of mind. As I say I'm sure it is absolutely nothing to worry about so try not to stress too much (easier said than done I know) these things are rare in your 20s I was told I was very unlucky! But even if it's something like a polyp these are easily removed! 
 

best of luck!