hypoxia... but why??

I've been having episodes of being short of breath, and on being admitted to a hospital they told me my pulse oxygen saturation dips down to around 85~75% percent sometimes (which is the doctors said was not a good sign), and so I carry small oxygen cartridges around with me now.

The thing is I don't get what the cause of this is.

They tell me its related to my anxiety, but all the information I have on anxiety and anxiety attacks tell me that the fight or flight reflex should be making me HYPERventilate, the complete opposite of HYPOventilation, which is what's happening with me. What's going on??? Please help.

[National Jewish Health]

Your low oxygen levels are not caused by anxiety.  You did not state whether the fall in oxygen saturation was while awake or asleep.  If while asleep, you have a sleep disorder.  If while awake you most likely have a lung problem or conceivable a heart problem with shunting of blood.  The low oxygen levels are most likely caused by the same disease that is causing you to experience shortness of breath.  You are correct that the normal response to a low oxygen levels, also called hypoxia, is to hyperventilate, breathing harder and faster.  It is possible, but highly unlikely, that you have a problem with your respiratory drive, causing you to hypoventilate.  This can easily be sorted out with a simple breathing test, called spirometry, a chest x-ray and measurement of arterial blood gases (ABGs).

Your doctors should be able to sort this out and discuss the likely causes of your shortness of breath.  Perhaps this would be a good time to seek consultation with a lung specialist to check your lungs for blood clots or chronic obstructive pulmonary disease (COPD) and your heart for a defect that allows poorly oxygenated blood to be pumped to the rest of your body.

Do not delay.

Good luck.