And yes, FLEX works with CPAP as well as APAP.
You can enable FLEX without changing pressure - it may let you know whether its the inhale or exhale pressure that bothers you.
The advanced FLEX machines have a few other capabilities, like reducing pressure just before inhalation, after inhalation, so that as your body changes mode the machine waits for you to get mid-cycle (if I understand the literature right). Personally, these features never seem to sync up with my body ...guess I am an erratic breather... so they don't work for me today. But its worth looking into if you struggle with keeping the mask on.