What Causes On And Off Bleeding From Buttocks?

i am a 18 year old girl, am not sexually active, but I do exercise a lot. running, mostly. But for the past month or so I've been noticing blood coming from my butt! Usually its just a tad of red on the wipe but sometimes i have to put a pad on to keep the blood from getting on my clothes. it is painless, and not continuous. i thought that whatever it was had gone away but today it was bleeding again! so much that i had to put on a pad bacause it was getting on my underwear :( im not sure what this is, if its serious or not, and im super embarased to ask my mom because she might want me to go to the doctor's office and that would be terribly awkward. help, please??