Is it possible for a 79 year old woman to be diagnosed with heart failure with preserved ejection fraction if less than a year ago she had a whole bunch of tests before a spinal surgery & all tests were good? she only has had high blood pressure meds?

She most likely had diastolic dysfunction for some time brought on by prior hypertension. This resulted in a thick,stiff left ventricle which doesn't relax during diastole impeding blood flow from the left atrium causing backflow into the lungs with eventual congestive heart failure despite a normal (forward) ejection fraction by echocardiography.

Yes, due to variability of testing and new onset conditions, such as arrhythmias, many times hard to catch and able to create the premises of sudden decompensations. A cardio consult and followup would be needed in such cases.