I'm with Opus on this, kitty needs to see a new vet for a second opinion. She should be well on the way to healed by this time in my experience having had 9 cats spayed over the years.
is the area red and swollen looking or oozing any liquid matter? the would mean its infected and therefore the stitches not holding? I think you do need to take her back for the vet to have another look at and question him as to WHY this is happening.....there shouldn't be any charge if his job isn't holding...unless there is an infection present of course.
Or if you feel better abt it take kitty to a new Vet, especially if the area is oozing or red, someone needs to take a look at it and find out why this is happening.
good luck, please keep us posted♥