First of all, I don't buy the hormone crap. Until we women stop blaming our hormones on bad behavior, we're never going to get anywhere. I dated in my 20's and I would never dream of being rude and ignoring my date. Frankly, her being on her phone at your house when it was just the 2 of you was rude unless the reason for the phone call was for someone to tell her that her mother had just been rushed to the hospital or something equally as emergent.
NHdisneylover may have a point Stu. What they posted may be spot on. Or your ladylove may be a tease. Or she may be immature. Or she may not be able to handle a relationship. Or she may be a lot of other things.
But the bottom line is that
you're the one who has to decide what
you are going to do. Stop walking on eggshells and make a decision. Either you guys are going to be friends, at which point you determine that her flirtatiousness is just "her" and you don't put any stock in it that it means any more than harmless, meaningless flritation on her part. Or ask her out on a date. A real date, where your intentions are quite clear. And I don't mean your intentions being getting her into bed. I mean "I like you, I want to pursue a relationship with you. I am willing to risk our friendship because I think you are worth pursuing a relationship with". That, of course, is if you think you want to have a relationship with her, as opposed to a hook up. You probably ought to decide that first.
If you decide you want the relationship route, I'd caution you to pay very close attention to her behavior. You say she's 21 and immature. Are you sure you want to deal with all that? You say she's moody? Are you sure you want to deal with the drama of moods changing from hour to hour? You say that you don't know how she'd handle discussion about difficult topics...well, discussing difficult topics is part of any relationship, so she had better be able to discuss them. If you guys end up together, and if you end up together for the long-term, there will be difficult discussions about finances, health, children, housing etc. Yes, she could be immature because she is only 21, but she could also be a diva, so pay close attention. There is a difference. I know the physical aspects of relationships are very compelling to a man your age, but don't get embroiled in something that will be difficult for you to get out of later based on ohysical attraction alone. I am watching my 28 year old nephew struggle with an extraordinarily difficult woman that eh has been dating/living with for about 2-2.5 years....let me tell you, it's not easy, so go into this thing with your eyes wide open. You have a right to be treated with respect, just as she does,
Traditionally, Stu, it's the man who sort of makes the first move. In this case, since she is young and since she doen't have much dating experience (having been in a long-term thing) that's what she may be waiting for and expecting.
The one thing that keeps popping into my head about this thing Stu, since I recall you posting about this girl in the past, is that it shouldn't be this hard. During my dating years (and I do remember them!

) some of the fellas I dated were nice, but no spark, some had a spark, but being with them was hard....lots of angst, lots of worry, lots of wondering about things like "Did I do the right thing? Did I say the right thing? Did I say something that angered him?". When I met DH, it was very easy. By that I mean that I didn't wonder if he'd call, because he called. I didn't worry that I said something that upset him, because if I did, he told me about and we discussed it and fixed it. It was just different with DH than it had been with other men I dated before him...we clicked without a lot of angst.
I know lotsof people are into "drama" today, but let me tell you, I've been married for 18 years and there's enough drama in life without adding to it by involving yourself with a Drama Llama.
So you have a lot of thinking to do. Good luck.