Pain just under belly button and immeadiately followed by bowel movement that is thick and mucusy and dark reddish brown, what could this mean?

Abdominal pauin. Symptoms and coloration of stool suggests bleeding irritation to colon. Many causes including colitis and chrones disease. Serious conditions requiring evaluation by physician. Also consideration include infectious causes amoeba bacterial and viral again require physician ands likely colonoscopy.
SEE A GI DOCTOR. Those symptoms could represent a lot of things and blood is an alarm sign that should be investigated.