Why haven't I had much of an appetite lately?

Was that food perhaps a bit greasy? That may have altered the pH in your stomach and made you go off food for a short period of time.

Maybe you’ve been really lazy this weekend and your body isn’t in need of your usual calories. If you are not losing tons of weight or anything like that, I wouldn’t worry so much about it.
You could be developing a stomach bug, or maybe you didn’t cook something well enough and got a mild case of food poisoning.
But don’t be so surprise. Anywhere after 25 years of age you will start to discover your body has a mind of its own and as @ZEPHYRA suggested certain chemical imbalances can start to show. I didn’t have acid reflux till I was in my early 40’s but I’ve know people to develop it in their 20’s.

But do pay attention to your body. Things that you could ignore in your younger years can be a problem or a danger to ignore as we age.
Your body may be trying to tell you that it is time to pay closer attention to what you are eating and not to just rely on a quick metabolism to magically erase an unhealthy eating style.

Even thin people can develop unhealthy body fat, and cholesterol or heart problems. Especially if they are thin and tend to be inactive.

Hopefully you put the leftovers in the refrigerator and ate them for lunch. You would have saved yourself about $3.
It is great that you know when and how to put the fork down. Many people can’t do that.
Congratulations!

You might be coming down with something. How do you feel otherwise?

A few days is probably nothing to worry about but if it goes on longer than a week please see a doctor. It could be a symptom of all kinds of things from a simple virus to Cancer. Only a doctor can help you figure it out.

I would see a doctor. Perhaps you’re coming down with the flu.