This needs to be attended to quickly. You are setting up that eye for glaucoma or a major blockage of the artery or vein in the eye which could cause permanent damage or blindness. Absolutely essential you see an eye MD ophthalmologist and not a non-physician optometrist (OD) for this problem. Also see your personal physician for a complete physical including blood work, blood pressure check, a test called CRP and sed rate and discuss a carotid doppler. Also discuss your CPAP. Technicians should check fit and pressure settings, use a small of mask as will work. You are going to need to learn to sleep without pushing your face into pillow which raises pressure in eye. You can try elevating head of bed and not using pillow. This is a potentially serious problem treat it as such with an Eye MD ophthalmologist and your family doctor or lung doctor.