As the behavior started after the spay surgery a trip to the vet is in order to ensure she does not have an infection. She won't go in other places of the house because she thinks this is the best place to let you know there is a problem.
It may also be possible you took the kittens away too soon. If they were born on T-giving and you just had this done that means the babies were only 6 weeks old at most! It may have been to soon and this is her way of showing that she is most unhappy with your behavior.
So first, get her to the vet to make sure it is not a physical problem. Then use the other advice posted here to try and change the behavior if there is no medical reason for this.