Period 5 Days Late

So my period is 5 days late and I want to know why? I took 2 pregnancy tests and they came back negative, but in a way I didn't even think that was a possibility considering I'm a virgin. Even if my boyfriend and I have done a lot of messing around we have been careful. I have noticed something odd, when I use the restroom I have noticed blood in my stool. Anyone know what's wrong?