Poor maddie
I dont have any experience with spaying a dog as my Jess was spayed when we got her and Poppy is still to young.
But I can give you this input about internal stitches...
Last year I had an operation to remove a birthmark on my leg for medical reasons and I had dissolving stitches both internal and external.
I had both because it was on my shin and they needed to do two sets of stitches to make sure it didnt open up because there isnt much give on the skin on your shin so it was very tight.
Anyway it opened up and started to weep and I was told that it was because the external stitches heal much quicker then the internal ones and sometimes they can open up because both are not fully healed.
It did eventually heal it just took a bit longer. (I know this dosnt have much to do with spaying a dog lol but maybe the stitches heal in the same sort of way Im not sure) Just make sure the area is kept clean so it dosnt get infected and it should heal quickly
