This is normal, and I too have twice smaller stools in diameter, than I had before issues. Two doctors confirmed it is due to spams and pain during BM.
Internal sphincter(bastard) is controlled by autonomous never system, which means you cannot relax it even you try very much. One option is to stretch - dilate it. Probably you can try this.
And also sphincter starts to spams when some feces get on wound, and nerve ending are irritated, it is not painful for the wound, but muscle reacts and starts to spasm.. Stupid, but it is how it is.