4 Weeks Ago I Had A Prolapsed Lower Intestine Repaired.

4 weeks ago I had a prolapsed lower intestine repaired. I was on Milk Magnesia fit two weeks. Now I am on one dose of Miralax a day. I don't feel my stools are moving like the should, my bowls feel heavy, but I am not successful at emptying them. I pass some stool everyday, but not enough. My stools are very dark, almost black, they are soft. My question, do I need to be concerned about the color of my stool? Do I need to increase the Miralax or add something else?