Blood in my stool once every 12-15 days.

I am under weight 47 kgs,have been this since my teenage. I have no medical record of serious illness or any anal or gastral problem of this sort. I have been experiencing anal bleeding through passage of stool for quite a while. Although its not that quite frequent as I experience it irregularly with atleast 2-3 month gap in between but currently it has been more frequent. As the blood was bright in colour, I conclude it to be from lower end of the anal cavity. Currently I lose few drops of blood although it was not the same before and when it happens I do not experience any pain. What kind of treatment would be recommended to understand the core of the problem as my diagnostic who checked my stool did not even recognise the problem until I mentioned it (as there was no trace of blood on my sample). Please recommend tests that could reveal the real cause of blood like biopsy, colonoscopy etc. as I have distrust over bogus medication since the diagnostic report (no offense).