As the digestive system is in proximity to the core muscles, any digestive problems may cause tension in these muscles. The problem is usually caused in the lower abdominal muscles as the large intestine is where junk sits and ferments. In sever cases one will feel cramping but in others it may be unnoticeable to the person. By looking at her posture in the first row of pictures, it looks as her abdominals are flexed which is causing her back to arch.