14 year old cat, weight loss

[lunabear]

Hi,
last time i brought simba for a check up i know he had some type of kidney problem, and was told to weigh him occasionally to make sure he hasn't lost any significant weight. he's been fine for a while, but i came home after the weekend (visiting the bf) and he looked really tiny. at the doctors he weighed a little over 7lbs and now he weighs about 6. this past week he's thrown up, just clear liquid. his diet hasn't seemed to change and from what i can tell he is peeing and pooping fine.
i do plan on making an appointment with the vet, but was hoping to hear some thoughts on what you think may be wrong.