Cause of anal bleeding?

A couple months ago after i went to do a number 2, for lack of a less embaressing word, i found blood on the tissue, the next day i had my period and just took it as a mistake on my behalf.
But other the last week there has been more blood and a sharp pain whenever i go to the toilet, i've just ignored it but ive been quite itchy down there and more recently feel..not pain but uncomfort when just simply sitting down. I'm only 17 and obviously don't have anal sex but im quite scared because isnt this a sympton for cancer? I know it sounds stupid but even with this knowledge ive still embaressed to ell anyone or go to a doctor!