Is there any relationship between the usage of CPAP and leg swelling?
Evidently her heart can't handle the pressure of air coming in or something causing her legs to swell.
Please see a cardiologist.
Sleep apnea is sleep disorder in which breathing stops and starts. If you feel tired after you have been sleeping whole night it is possible that you suffer from sleep apnea. There are two types of sleep apnea: obstructive sleep apnea and central sleep apnea but it is possible to have combination of both which is called complex sleep apnea.
Treatment is necessary so you can avoid problems with heart in the future. If you are suffering from a mild case your doctor will recommend lifestyle changes such as quit smoking and losing weight. If this is not enough you will have to wear ACAP and this can cause additional complications. You need to see your doctor as soon as possible.