Hey mystang,

There are several things that could cause lack of appetite. It could be from medication if you are taking any, simply being upset, nervous, lonely, anxious, or depressed could cause it.
It could be from an acute infection, but it is likely that you would have other symptoms in that case. However, the change in bowel habits supports the infection possibility. Your physician could rule that out by analyzing some stool samples. In the most serious cases it could be a sign of a developing disease such as cancer. Because of that risk it is important to rule everything else out with fecal testing and possibly a colonoscopy.

This is a very hard thing to diagnose without actually seeing you. If it persists you really ought to see your physician and get some basic blood work done.

The length of your symptoms may range from temporary (if it is caused by a virus i.e. gastroenteritis) to longer if something more serious is found. Since you are a generally healthy person it will probably pass soon.

I hope this helped and I hope your symptoms go away with no other problems!

~Chris