It could possibly be a behaviour problem following her having the kittens. If this is the problem hopefully she will start to encourage them to become more independent in the next couple of weeks.
Having said all this, I would certainly take her to the vet. If she is stressed about her kittens, this could well bring on cystitis or it could well be she has an infection. Both need medical treatment. If cats have an irritated bladder all normal logic goes out of the window to use the litter try, they just have to go there and then.
Either way, I would certainly get a Feliway diffuser which in many cats helps to relax them. They take about a week to help, but in either of the above would be well worth ago. Your vet probably sells them or you can get them online cheaper from places like Vetuk, Zooplus, MedicAnimal.
Do you have any large sheets of plastic you can put on your beds? It might divert her somewhere else, but at least you haven't got to keep washing covers/quilts etc.