It depends how bad it is and how far along or how long she had it. I do know when you are lightheaded and dehydrated and get an IV to hydrate you, you feel better pretty quick but there is no way she should have been doing anything except resting. Of course, imagine if she actually did that like she was supposed to. The story/narrative would be that she is halfway to her deathbed because of not showing up for a 9-11 event so frankly I would have done exactly what she did. She loses much more by not showing up at all versus what happened.
As far as believing it, none of us know the full truth. Whether that's what she really has, whether there is something else with it, the severity, etc are all seemingly dependent on people's political leanings, especially in this thread.
Her known symptoms (bad lingering cough, dehydration, lightheaded/fainting) are all symptoms of pnemonia. That's all I know.