I wish I could be more helpful, but I don’t have enough information to give you a clear answer. What I can tell you is that there are many, many reasons a 20-year-old can be moody. It doesn’t necessarily indicate a mental illness. It may mean that there are stressors in her life that she isn’t handling well. She may have an undiagnosed medical problem. It may have something to do with her feeling like it’s time for her to move out and move on with life but perhaps she doesn’t have the resources or support to do so. It could be that there is a problem with a relationship. And, of course, it is possible that she is showing signs of emerging mental health problems.
The best way to find out is to ask her with as much gentleness and support as you can — even if she is making you angry. Being angry with her won’t help. Being compassionate and offering your help might.
I wish you well.