I've had the same problems too! im 17 years old and its just been happening in the past year. It always happens to me in all the wrong moments expecially at school, and I have to rush to the bathroom. I hate having this whatever it is... i hope there is somthing that can fix it.
the mucous is the main confirmation that you have ulcerative colitis.
You need to see a gastoenterologist, who will want to confirm this with a colonoscopy. there are specific meds for this, all RX - like Asacol or Lialda that will make life bearable again. In the meantime, try Activia yogurt every day, and a supplement of Aloe Vera gel capsules made by Pharmassure - available at Rite-aid.
When u get the answer to this question please let me know I have had the same situation. I was once told that it is because of the bowels being infected and have actually taken antibiotics for 30 days to help correct problem with bowels, they were bactrim 500 3x day and flaygyl 500 4x day.....this helped for about a few months then right back to the same ol same ol......good luck