Checking for worms like Karen suggested is a good thing to rule out. I would hope that for a young cat her kidneys would be OK, and certainly her disinterest in drinking doesn't seem to suggest that she's in renal failure.

I have seen a cat with early-stage heart failure presenting similar symptoms to Brandi's, although I wouldn't let this alarm you as there could be many other explanations. I am sure that if your vet suspects anything of this sort, then they would take the appropriate tests.

Good luck, and I hope you get to the bottom of it!