My cat's acting weird... Please help!!! She's not herself and I love her :(?

My cat is 15 years old. She does everything she always has (purr, eat, drink and sleep normally, and from what my mom says, using the box normally, and even going outside to play). I don't know, she just seems a little lethargic lately, kind of lazier. She does have CRF, but it's in its early stages. My mom suspects a UTI, but the littler box usage is normal (from what she says anyway). She has hyperthyroidism, but is on medication and seems okay with that. There is a vet appointment next week for her 6 month checkup (since she's been on meds). I'm thinking, that since she vomited yesterday and a little today it may be an upset stomach. She doesn't vomit much anymore since she's been on treatments. Any ideas please????? I love her and I think it's an upset tummy, but I don't know.