Shunting but no PFO

Six weeks ago, I underwent a transcranial Doppler with microbubbles.  There was no shunting at rest, but major right to left shunting during the Valsalva maneuver.  The cardiologist was confident that I had a Patent Foramen Ovale.  This morning I underwent a transesophageal echocardiogram.  No PFO was found!  The doctor was amazed.  He will be sending me for more tests to find the "leak" but I have been disqualified from the medical study I was participating in.  Any ideas where the shunting could be coming from?  I thought I had a PFO and had nothing to worry about.  Should I be worried now?

[Cleveland Clinic]

A PFO  is only one of the many possible connections between the Right and left systems. You could have an AV fistula, or a pulmonary vein that empties to the right atrium. Hence a CT angiogram of the chest should give than answer in most patients, for why you are having microbubbles on a TCD during valsalve.
Another possibility is that you did not strain hard enough during the TEE because of sedation, so the shunting was never observed during that procedure. .