Sudden stomach pains and gas after drinking/ eating something a bad mucus comes out of anus, and a constipated bowel movement. what does this sound like?

Proctitis?: Proctitis is inflammation of the rectum, suggested by anal discharge and can also cause constipation and cramping. If "bad" mucus means it is large amount or colored, e.g. yellow or with blood, it's probably an infection. Several possible causes, but at your age, STDs are high on the list of possibilities, assuming you are sexually active and especially if you have anal sex. See a doctor ASAP.