Abdominal Pain in Teen: What Does a Negative Appendicitis Test Mean?

Question
My teen daughter was vomiting and had abdominal pain which moved from around her belly button to the right lower quadrant of her abdomen (she rated it 4/10). Because she also had a low grade fever but no diahrrea, our doctor recommended that she be checked out at the ER (it was late Fri eve). An x-ray, an ultrasound and a CT-scan later, the doctor told me that they couldn't find her appendix!  She had a normal CBC so many hours later she was sent home.  I should add that, while we were in the ER, her period started.  She says she still has the pain in the same spot but less so.

Is it possible that she doesn't have an appendix?  Should I be concerned that it could still be appendicitis even if so many tests couldn't "find" her appendix?  Thank you.

Answer
those tests are not even close to 100% in diagnosing an appendicitis let alone always seeing it.  She has one, and yes, it still could be appendicitis even with neg tests.  If the pain improves over the next few days then it isn't.  It is esp hard in women with ovarian cysts etc complicating the situation.  Hope this helps...