Cat Peeing on Dog's Bed?

My cat, Lilly, and my mom's cat, Chester don't get along very well. The two don't normally fight, unless Lilly gets close to Chester's kittens who are 3 weeks old. They both love our dog, Hunter.

Lilly has been peeing on Hunter's bed since she went into heat. Her first one with us (she was a street rescue and we just haven't gotten around to getting her spayed), was in the beginning of January (it's currently the ending of March), so, her heat schedule is normal. We will be getting her spayed after she is no longer in heat. Her swelling poses a risk. I think is too dangerous to do it now.

My question is, why is she peeing in the house? We have 3 litter boxes for the two cats to use, but Lilly still pees on the dog's bed.

By Cassie S.