I get the abdominal weakness as well, including pelvic floor and sphincter muscles, especially when the MG gets really bad. I usually do not have the strength for a bowel movement until I have had my morning mestinon. After ending up in the ER with shortness of breath and released, I did not have much control of my bladder for 2 days. This has been better since I have been on pregnisone.
Sorry you are having the same trouble
kathie