Cat is eating less

Siamese cat, 1.5 year old female, spayed, 7.5 lbs. She has no other medical history. Only animal in the household, indoor only.

Noticed symptoms over past 4-5 days. Located in northeast USA.

Basically, I have noticed she is eating less than normal. She still begs for food in the morning, but only eats about half of what I give her. In the evening, she doesn't eat her food at all or even get excited when I give her the cue that it's dinner time. I took her to the vet last week because I noticed she had suddenly started drinking water, which she has never done in all the time I've had her, and was eating a little less than normal. They did blood work and urinalysis and said everything came back fine. But now she's eating so little that I'm worried she's going to make herself sick.

Would appreciate any advice, thanks.