Covid-19 symptoms coming in two waves is something I've heard from a few people, and anecdotally fits my experience - generic symptoms (body aches, fatigue, fev

Covid-19 symptoms coming in two waves is something I've heard from a few people, and anecdotally fits my experience - generic symptoms (body aches, fatigue, fever) cleared up, then about a day later my blood oxygen levels dropped sharply with no obvious symptoms (happy hypoxia). I wonder what causes this apparent pattern. I had a positive PCR test during the first set of symptoms FWIW.