You need to let your doctor know about this. It may be related to your transplant...it may not. But this isn't normal so contact your doctor to find out what is going on. Keep yourself well hydrated as well.
Welcome to the gastroenterology community! What do you mean by "lasts for 6-8 hours" (are you sitting on the toilet for 6-8 hours; are you having leakage that lasts for 6-8 hours)?