Please help! Cat won’t stop peeing on my bed!

Hello! This is my first time posting here, and I’m on mobile, so please forgive any errors. For the last few days, my cat has been peeing on my bed. I immediately took her to the vet and they did a very thorough exam, tested her for a uti, and found nothing wrong. She’s ten months old, and has never had any kind of issues like this before. I have another cat who is 22 months old, and they’re absolute besties. They sleep together, eat together, groom each other, and share everything with no problem. I already did some research and found that she could be feeling insecure in our bond, so I went from brushing her once a day to twice a day, increased the amount of time I play with her, and made sure we get more time alone, but none of it seems to have helped. I have 3 litter boxes, and I’m very diligent about cleaning them every day. The only thing I’ve changed is the litter they use, but I made the switch two months ago and still use the same brand. I’ve also had the feliway diffusers since we got her. She hasn’t had any noticeable changes in mood/activity level/demeanor otherwise. I love her so so much and hate to think of her suffering!! But I just cannot deal with the peeing in my bed. Any and all help is much appreciated!!