What's been suggested already are very good tips.

Only thing I can add to it is to take a urine sample to your vet. Several of my cats had infections we didn't know about until we did a urine sample on everyone.

It's not that hard to get a urine sample from a kitty, yours sounds even easier than mine since yours seems to have a schedule. Just keep a low cup handy, when the kitty goes out, or goes in the sandbox just slide the cup underneath. You might get a few stares and your hand might get a bit wet, but you'll have your sample.