My seventeen-year-old DD does this sometimes.
She goes from being very nostalgic and upset about missing all the lovely things she did as a child to getting very excited about the things she will do in the future.
I think the nostalgia about childhood definitely, in part, comes from a place of anxiety when the practical aspects of living were sorted and organised for you, when you didn't have to take responsibility for your own actions so much, where you didn't have to deal with uncertainty and being out of your own comfort zone. It's fear of the unknown.
The trouble with Covid-19 and how things are currently, is that teens and young adults have had the breaks put on their lives, so many of the good things about being older that balance out the childhood nostalgia and fear for the future, such as independence, freedom, friendships, travel, first jobs, first flat share, meeting potential bfs/gfs etc, driving lessons, etc have stopped or paused. And this is making adult life very unattractive currently as their wings have been clipped and they are left with the more boring bits of being an adult, and none of the excitement.
Your dd has extra challenges which must make things particularly stressful for her, and I don't know how autism affects the way someone looks at the future for example, but I honestly think, in normal times, veering between nostalgia for the past and excitement and trepidation about the future is pretty much a definition of what it is to be a young adult or late teen, if they have had a reasonably happy upbringing as a child that is.
What seems to help my DD is discussing the happy times we had, looking at old photos etc but then going on to discuss all the exciting things she has to come. And letting her vent all of her current frustrations which she does rather vigorously and at length, usually directly at me 
I guess the main thing to watch out for is when stress and nostalgia turn in to full blown anxiety and depression and whether these things are interfering with her daily life. At that point , asking for support might be helpful. 