Blood/mucus in stool

Hi I'm 17 and about 4/5 months ago I had a small amount of blood in stool. The blood was kind of mixed with mucus but it wasn't mixed with the stool. And 2 months after that I had the same thing so I went to the GP and he said it's most likely to be hemmoroids or anal fissure. So I wasn't that worried but last month I had it again but there was more blood then before, again it was mixed with mucus but not the stool. Since then I've been really really worried and I don't know if it's anal fissure or something else serious like cancer. Can someone please tell me what it could be.