Hello! Firstly, well done for supporting her in getting a diagnosis, accepting it and looking for ways to understand/ support her. It sounds obvious but you'd be amazed how many autistic children (and adults!) do not get this from their families.
In terms of being cooped up, has she talked to you about how she feels about this? It could be that the demands of the compulsory social interaction at lectures etc are enough and she really needs this downtime and space to cope. Or it could be that she feels isolated and lonely and doesn't know how to change this and is struggling to adjust to the huge change of living away from home, living with strangers, etc. Very important to talk to her about how she's feeling if she will open up to you.
Also, does the university have good pastoral support? Do they know her diagnosis? They should be helping, making adjustments if needed. Can probably connect her with other autistic students if she wishes.
Being autistic and living in a world not designed for you is hard. We are all different and each struggle with some things more than others. I hope her diagnosis report gives you some useful information on which areas she's most likely to struggle with and what kind of supports she may need. Many of us do manage a lot, but it often comes at a huge cost, with burnout and damage to mental health. Masking and desperately trying to fit in does a lot of damage.
What is positive is that now hopefully as she has a diagnosis and is still young she can be kinder to herself. It's a lot to process but it can really help, to accept yourself and instead of "bashing your head on walls" trying to do what everyone else does and thinking if you just try harder you will be able to, you can instead forgive yourself and focus on finding a niche that suits you.
I am often overwhelmed because I designed a life for myself that is overwhelming, before I knew I was autistic. Children, demanding job, and now I am stuck with no way to reduce those demands. Some autistic people might cope fine with that, but for me it is too much but now I am trapped. Your DD is young and still has lots of options so she can hopefully find a life that will work for her and enable her to be content and happy.
Beware of the "swan gliding on the surface while it is desperately paddling underneath to keep afloat". Many autistic women reach burnout and have mental health crises in late teens/ early 20s when life becomes just too much. She is lucky to have a supportive mother who is looking for ways to help her.