I'm no expert but it sounds to me like your not consuming enough calories. When this happens you can go into starvation mode and your intestions can lose the strength it takes to move your food through your system (peristalsis). This happens due to muscle wasting.
Your remedy is to simply start eating more calories, and slowly you'll gain this strength back. Many anorexics go through constipation such as this during refeeding. They may not have a bowel movement for 6-10 days, and they are not given laxatives.
It will go away, but you must eat. Theres no other way out of it. Laxatives and extra fiber will only create more problems. Get the recommended amount of fiber, calories, and eat balanced meals. You'll be okay and never have this problem again.