Have you ever had your kidneys checked? Because there could be something wrong with them, they produce this substance called erythropoetin which promoted blood production by the bone marrow
Plus since you have lower back pain
, which is sometimes associated with the kidney it could be that you're kidneys have some problem.
The bloody stools could be because of haemrrhoids, but without any tests its hard to say.
I recommend you see a doctor right away because for an anemic person to lose blood is a very dangerous situation.
After running some tests they can narrow down the cause and accordingly treat you for it.
I hope this answered your question.