thank you for your query.
your symptoms suggest some sort of gastrointestinal bleeding
the jet black colour of the stool is due to the blood that gets digested.
since the blood is in stool most commonly the bleeding is from the lower part of the intestine.
Lower gastrointestinal bleeding is typically from the colon, rectum
5.usage of medication like NSAID's,SSRI's
since there is loss of blood and continuously at that you are getting the additional symptoms of dizziness
,loss of appetite and cramps
i suggest you to kindly get a colonoscopy
done by your GP to know the exact region of bleed.also get a complete blood picture and also BP checked this will rule out infection.
we can start anti-biotics and other treatment accordingly.
wishing you good health