My daughter began wearing her plastic retainers less than two weeks ago (she had metal braces), and yesterday she awakened with very white, swollen lips! After having a private freak, I realized that she was dehydrated. After she drank copious amounts of water, her lips returned to a normal size. At first I thought she was having an allergic reaction to the retainer plastic (but that would be unusual after wearing it round the clock for almost two weeks).
She was often thirsty the first week after beginning to wear the retainer, but slacked off on drinking water as her thirst diminished. After all of this, it might not hurt to keep drinking as much water as is comfortable after your thirst diminishes because even if you don't feel thirst, the drying effects of the plastic might still be there. Of course, I am not a doctor, but if you could have seen my poor baby's mouth... She is perfectly fine now, by the way (and drinking lots of water again!)