I have symptoms of appendicitis, but my ultrasound comes out normal. What else can it be?

I think I have all the symptoms of appendicitis (except the part where it bursts). It has lasted about three weeks now. Doc told me to do an ultrasound and everything came out to be normal. Now he told me to get a CT scan. My blood test also shows everything is normal except LDL is a bit high. What else can it be if it's not appendicitis? Thanks.