I also use a product the same as columbine, changing my litter quite some time ago, there is minimal dust, and it is a rather good and economical product ,its called breeders select here in NZ, and is recycled paper pellets with no additives or chemicals.

However i do think the incense could be contributing to the problem, if my daughter lights any in her bedroom it affects my lungs terribly, i do not have asthma, but am a chesty type person, and have sensitive airways.,maybe your kitty does to.