Oh I am so sorry to hear!!!
I would also go to the the V-E-T asap...

If there is no medical reason, maybe Pixel has once been disturbed by Randi while she used the litterbox...maybe a change to a hooded box will then help or replace the box?!
Or maybe you should try a new cat litter brand...sometimes they decide out of the blue that they actually don't like the other stuff anymore...

However good thoughts are on the way to little Pixel!!!