Thank you for the query.
The most probable reason of your sufferings are hemorrhoids. It would explain blood in stool
, lump and anus itching. But before such diagnosis some other blood in stool reasons should be ruled out. As you do suffer from sacroilitis I assume that you do use a lot of nonsteroidal anti-inflammatory drugs. Such medicines can cause peptic ulceration what can cause blood in stool as well (such blood is dark or black). Moreover, blood in stool can be caused by colon cancer
that is why it should be ruled out as well.
In conclusion, if you do suffer from upper abdominal pain
, acid reflux
, nausea, and the blood in stool is dark or even black, you should have upper GI endoscopy.
If your blood in stool is bright or red and mixed with stool, you should have colonoscopy
If this bright or red blood is only on the surface of the stool it is most probably caused by hemorrhoids but colonoscopy is still advisable as such blood in stool can be caused by rectum tumor as well.
Hope this will help.