H.E.
My twins were born at 37 weeks, and seemed fine at the hospital. The day after we came home, we went back for a sleep study and they put him on an apnea monitor.
They also said they had no idea why and ran all kinds of tests, which all came back without answers.They finally decided it was probably that his body reverted back to breathing how he did when he was breathing amniotic fluid rather than oxygen. He did outgrow it, but had the monitor for a month.
It's not a fun thing to deal with, and I feel for you. Hang in there, it's wonderful to finally be done with it!