The tightness around the umbilicus
is because of swelling following the operation because the surgeon put some stitches inside to close the port site, which are usually dissolvable stitches, but it takes atleast 3 months to dissolve. So it will resolve with time, if not then you should get an ultrasound
done to exclude any port site hernia.
For decreased appetite, you need to make sure with the surgeon that whether the appendix
was inflammed (appendcitis) or not, if that was not the case and appendicectomy was done regardless (which is not a wrong thing to do), then other intrabdominal causes like irritable bowel syndrome
or inflammatory bowel disease
etc should be excluded. So try first try yourself to eat whatever you feel like, in small frequent diet and monitor your weight, if not improving the get yourself checked.