Cat randomly pees on dirty clothes

My cat has twice peed on dirty clothes, once yesterday and once about a month ago.  The first time she jumped into the hamper of dirty clothes, and the second time it was on a few pieces of dirty clothing on the floor.  Both times were in the bathroom at night as we were going to bed.  She seems to be healthy otherwise.  I don't think it's a UTI or anything because she's only done it twice, not consistently.  Our other cat has never done this.  She's never done it before, either, even though she's always enjoyed (for whatever reason) rolling around in the basket of dirty clothes.  There's not been any major changes in our lives.  We've all been a bit more stressed than normal with career problems and such, but nothing really major.  Any thoughts?  

Welcome to TCS. What kind of litter are you using? How many boxes do you have? How often do you scoop the boxes? Does the other cat 'guard' the boxes?

I don't know the brand offhand, but the litter is one of the walnut-shell brands.  We've been using it for quite a while now.  We clean it every day.  No, the other cat doesn't guard anything.  Except his treats.    He's scared of everything, but otherwise is very mellow.  There's no variables that have changed recently, so I can't come up with anything.  SHe doesn't seem to be avoiding the box or anything, as it's only twice in a month or so that she's done it.  

She was going to do it again last night.  We saw her sniffing around in the pile of dirty clothes, and I took her to the litter box and set her in it, and she immediately started peeing.  I just don't feel like it's a medical condition, because if it was, wouldn't she do it every time, not just right as we go to bed?  It's always the same time.  As we go into the bedroom, she follows us in and goes into the bathroom most nights.  The rest of the day she's fine.  I'm confused and don't know what to do.

Since she is doing it only on the dirty clothes (the ones that smell most strongly of you) she could be adding her scent to tell the world "These people are MINE"

But why all of a sudden?  We've had her for 6 years, since she was a kitten.  And how can I keep her from doing it again?  I really don't want to be doing laundry every night at midnight.

See, you didn't tell us she had been yours for 6 years. That changes things. Even though you don't think she has a UTI, the best thing to do is take her to the vet for a checkup. Cats are good at hiding pain and stress.

Another suggestion is to not leave any clothes on the floor and not leave her any way to get into the dirty clothes hamper.

Have you noticed any change in the smell of the litter box? A minor change in the litter you use?  Even a subtle change can trigger avoidance, including just occasional peeing on the clothes.

If it was a UTI, wouldn't she avoid the litter box all the time?  

The only time she ever does this is when I'm going to bed.  She follows me into the bedroom and starts looking for a place to go.  The rest of the day, she apparently uses the litter box, because I've never found or smelled urine anywhere in the house the rest of the day, and there's always 2 cats worth of stuff in the litter boxes.  It doesn't make any sense to me.  There are clothes or clothing-like items (pillows, blankets, slippers, etc) lying in a few places throughout the house, but she doesn't ever try to do anything to them.  There's a towel on the floor behind me (she's frequently in this room) and she's never bothered it.  

THe only difference I can think of is our other cat is much more relaxed lately and doesn't seem as scared of her.  He used to be afraid of everything, but he's just kind of grown out of it, I guess, and isn't as jumpy and skittish as he used to be.  

We haven't changed anything with the litter.  Honestly, I try not to breathe much when around the box, so I haven't really noticed any smell change.  

I really don't know what else to tell you. I have exhausted my knowledge about the subject.