As navel is deeply related to our digestive system, any displacement in navel would surely lead to sour belches, indigestion, constipation, loose motions etc. and can also cause sudden weight gains and losses. It is similar to misalignment of the spinal cord and displacement of the navel has relation to weakening of ligaments related to large intestine.
If your navel is displaced, it should have been first brought into its position instead of doing agnisara kriya. You have mentioned that the rectus abdominus is as hard as rock. Rectus abdominus is group of abdominal muscles and muscles should always be soft, uniform and consistent. The only time a muscle should be hard is when it is in a contracted state (i.e. when it is actively flexed). During nauli, when other abdominal muscles are kept in relaxed conditions, only the abdominal rectii muscles are contracted. So, a tight abdomen in relaxed state may have many different causes (other than musculature causes), including constipation, irritable bowel syndrome, and food poisoning or even, as you said, swelling in the intestines can also be the cause. The causes of the swelling in the intestines can also be attributed to the navel displacement.
Continuing with agnisara kriya can aggravate the intestinal swellings. So, I would suggest you to first get your navel placed in position through yogic techniques, I have explained on the website (or get the help of qualified yoga therapist). Also go through the causes that have led to the navel displacement - like heavy weight lift by jerk or on empty stomach, etc and prevent them in future to avoid navel displacement. Also, work on your abdominal weakness to prevent the navel displacement, if it happens frequently.
Once the navel displacement is rectified and internal swelling of the intestines is healed, you can start the agni sara kriya, subject to the condition that your diet should comprise of enough roughage and include micronutrients. To boost the healing of intestines, you can also use the stimulating abdominal pack, one of the naturopathic protocols. You can also write back with your complete details (age, lifestyle - sedentary, frequency of displacement, etc. so that I can guide further in healing the intestinal /navel issues.
