Could it be because you are only looking for mental stress. What about physical stress? What shape are you in. Do you smoke? Do you have something debilitating, Diabetes, Hyper or Hypotention, Arthritis, or chronic fatigue? What about Lyme? There are a lot of physical, mental and complex things that can cause anxiety. If you are anything like me you block out pain and fatigue just to keep going so you may not notice how much anxiety it is causing.
Some times you can feel the anxiety build and you mistake it for just being tired. Some times if you are busy it sneaks in and hits all at once. But how it comes matters nothing if you have your relaxation and coping skills in place to handle it when it does. If over work is bringing it on and you can't or won't slow down then you have to know how to handle it when it does. Just knowing how to handle it will reduce the number of times it happens.
If you find what sort of unnoticed stress you have please let us know, I am sure others have the same problem and can use the help.