Black stool

[Flyboy1960]

I have had black stool on and off for a year.  I am seeing a specialist who doesn't know what's causing it. To combat the severe pain, enough to drop me to my knees and I'm crying from the pain the specialist prescribed lyrica.  It helps with the pain but does not get fix the underlying problem. Now and then any type of vegetable comes out of me black as road tar.  

I have had two tests come back.  One from the family doctors lab and one from the cancer lab both came back as bleeding inside my gut somewhere.  Yet the specialist because I'm not bleeding enough to drop his tests significantly keeps saying he doesn't know what is wrong.

Whatever is wrong is crippling me.  If I go out I have to be near washroom.

Can anyone give me ideas of what could be wrong?

Thanks