Why does my female kitten pee only on my bed?

Our kitten is about 8 months old and is one of 2 cats we have (other is a 2 year old male). We also have 2 dogs that play with the kitten but she's not a big fan of large dogs wanting to play. Within the last few months she has started peeing only on my bed. no other beds or furniture in the house. I have taken her to the vet and she has no UI problems or such. I keep 2 litter boxes in the house and they are typically cleaned every other day. Is there a way to get her to stop this behavior? I just bought a new matress thinking that the old matress was the reason she peed on it..but she has peed on the new one. (luckily I have plastic on the bed but do not want to keep leaving plastic on my bed forever)