Siamese cats are very prone to kidney disease and urinary tract infections. Sorry Sariah, you have got it the wrong way around - The males are more prone to it than females because they have "longer" urinary tracts (this is the opposite to humans where women have longer urinary tracts and are more prone to infections than men). If your cat has been to a vet then he has probably checked for this. Two tell-tale signs are:
1) small droplets of blood or discolouration in the urine
2) rather than lots of pee just a few drops

If either of these are present then you should get to a vet as soon as possible.

BUT...it does sound more like an environment issue - the dog? If the dog is an indoor/outdoor animal then it may be a good idea to keep her outdoors during certain times so that Simon can get some quality time in the house? How long has the do been around?

Troy.