What Causes Sudden Weight Loss With Frequent Bowel Movements?

i ve lost a lot of weight in the last 4 1/2 months and my appetite is the same but i have been having a lot of bowel movements. Sometimes i got to the bathroom 3 x a day or sometimes i think iam finished and I have to go again. I was taking concerta because i thought it was due to my weight lost but i am still losing weight. I used to be a healthy 132 now iam unhealthy 121...not good. Iam a naturally thin person but this is really bad