If she peed on the quilt and now the blankets, did you treat the mattress with an enzyme cleaner? One of the problems with peeing outside of the box is that once they go somplace, unless you completely eliminate the smell (to their sensitive noses), it remains a place to go to the bathroom as far as they're concerned. Blankets AND mattress need to be treated with an enzyme cleaner.
Because you have to soak the area of the mattress that may have been affected, place a thick (or folded over) towel over the area you treated and then put on your already cleaned (add enzyme cleaner to your wash in place of bleach) mattress pad, sheets and blankets.
We had a problem with a cat that began peeing outside the box, and the bed was one of the places she'd go. We retrained her by cutting three boxes down the side so they could lay out flat, and every morning we'd make the bed and then cover it with the cardboard boxes. We'd take them off before we got in bed. We did this for two weeks. In combo with enzyme-based cleaner treatment, this worked for us.
The thing to remember is that when kitty doesn't use the litterbox, the first thing that should be done is get kitty to a vet. 85% of the time they don't use the litterbox because of a health problem.
The early spay is not the cause of this.
If it is not a health problem, then it could be one of many things. I agree with trying different litters. And instead of just changing the litter, perhaps consider purchasing several more litterboxes and trying different litters in them. It could also be the location of the litterbox.
And if it's a covered litterbox, consider removing the cover. If it's an uncovered litterbox, consider adding a litterbox with a cover. It could be one of so many things - the litter, the type of box, the location of the box. It's easiest on you to purchase a couple of new litterboxes of different types and try putting different types of litter in each, and put them in other locations, and see if that helps.
You can try putting the litterbox on top of the cardboard on the bed. Do that for two days. Then put it down on the floor at the end of the bed, or to one side of the bed. Leave it there for two or three days. If she's using it, then move it half-way to the bedroom door. Do that for a few days. Then move it out the bedroom door, etc. Slowly move it to where it "lives." This is a method of retraining her to associate going pee with the litterbox, but it starts by placing the box where she's been going to the bathroom.
Also, one of the reasons cats pee outside of the box is anxiety. Especially if it's on your bed. This way her smell mixes with yours and an anxious cat can find that reassuring. If it turns out it's not a health problem, the litter, the box, the placement, and "retraining" doesn't work, then perhaps consider consulting with a cat behaviorist to help determine the potential source of anxiety.