So sorry that you are in so much pain. It is difficult to think straight when you are in that position. Breast feeding is dehydrating too, as you probably know, but I would not be able to give you advice on that, though I know that there are others here who had surgery while breastfeeding and hope they will come up with suggestions. Miralax seems to be the best stool softener, though I use MoM.
Do let us know how it went with the doctor.