What Causes Blood In Stools?

hi, i'm 20 yrs old. About 6 months ago i started seeing blood in the stool around 3-5 days before my period. It lasted for a couple of months each month. I checked with a doctor and she checked me for external hemorrhoids and there was nothing. The blood is bright red in color, and she told me to eat lots of fiber and drink lots of fluids and see if it goes away. If not, she referred me to get a colonoscopy. I tried what she told me and it seems like it went away for a few months. Yesterday on the other hand, i used the restroom normally,and after finishing i saw blood in the stool, i also have been feeling itchy in that area for the past few days, and i have had a lot of pelvic and lower back pain. I am not sure what it could be. I am due for my next menstrual cycle. If you would please let me know!!