What Causes Pressure In The Rectum After A Colostomy?

I recently had colon cancer surgery (back in July)...I have a colostomy...in the past couple days, I ve experienced pressure in my rectum and have expelled a brownish phlegm looking stuff the size of a golf ball from my rectum....it smells bad....I have increased pain in my lower abdomen (my diagnosis is stage 4 colon cancer or adenocarcinoma of the sigmoid colon)...I take xarelto for the blood clots that I threw after surgery, if that makes any difference.....also, the tumor enveloped my ureter, which had to be reimplanted into my bladder....I m urinating without difficulty....should I be concerned, or, does this just happen?