My girlfriend is extremely stressed from work/family but also having second thoughts?

So this girl and I have been best friends for the last three years. All of my friends and family love her and hers love me. We have always been extremely close and she would always come to me for stuff. We've been dating for 6 months, and it's been the best decision of my life to ask her out. We've made each other extremely happy and everything has been perfect, until recently. It all started when her ex-boyfriend/first love started contacting her. He broke up with her two years ago (he also cheated on her) and never bothered to contact her until me and her started dating. She was honest in telling me he had contacted her and I appreciated it and told her I trusted her and to simply not tell me when he contacts her because I trust her. About two weeks ago, she started acting very strange. She suddenly became cold, distant, and very dry with me when she was normally very cuddly and sweet. In response, it's made me act weird as well. I couldn't go any longer with her acting this way, so I confronted her and asked her what was going on. She told me that he had kept on contacting her and to make it short, it started messing with her head. Not that she would ever leave me for him, but she says that first love feelings are hard to get rid of. I was very upset and found it very unfair because I have done nothing but be there for her and I've always treated her well. She told me she would tell him to stop contacting him, which she did and they haven't texted since.

However, that has not stopped her from acting weird. I know that she has been going through a lot of stress with work and she has also been having a lot of trouble sleeping which she says has been making her feel depressed, but whenever she was stressed she would normally come to me and I'd make her feel better. However, the last two weeks, whenever we hang out I just get the vibe that she doesn't enjoy herself around me and that she's unhappy, something I have never seen out of her before. I went over to her house a couple of nights ago and confronted her and asked her what was going on with us. She replied saying she doesn't know what's going on with us. She emphasized her stress and that because of that her mind is just a blur and that she didn't have an answer for me. I emphasized how I have done nothing wrong or treated her badly and that it's not fair how she's been treating me. She still had no answer so I made it simple for her: She was happy with me and wants to work through this to make us work, or she wasn't happy with me. She still gave no answer, so I simply got up and stormed out of her apartment. When I got home she texted me saying:

"I'm really upset that you left like that. I care about you so much but I think we need to talk about things when we both calm down. I'm so sorry things are like this."

I was so upset that I have not replied. It has been two days since that text was sent. She posted something about how she had been crying all night and I am dying to talk to her. The only problem is that I truly feel she is taking me for granted. Unfortunately, the guys she has been with have all used her, and I'm the first guy that has truly treated her well and truly respect her. This is why I feel that she doesn't appreciate me as much as them because I am always so transparent and good to her. But I have now come to that point that if she can't reciprocate, then I can't give any more. I am invested in this relationship but I can't do anything if she won't invest. I want to talk to her but despite her having sent the last text I am hoping that by not responding she will come to me. I am in love with this girl but I feel that if I go to her she will feel as though I will always come running back, and I need to assert the fact that I am not playing games and that I will not get taken advantage of or be played with. Am I doing the right thing here? What should I do?

What weirds me out is that even as friends she would always tell me how I'm such an amazing guy, and whenever I had problems with other girls she'd tell me that they were missing out, etc. So why did this behavior suddenly come along?