Girl's Behavior

What did I do wrong and how do I fix it? Because I'm starting to really care?

So I (23M) have worked with this girl (22F) for over a year now, and for about 10 months we've been texting (sexting) etc. As time passed on we began to realise that our interest in eachother wasn't purely physical and we began to become really close and good friends. 2 months ago on the way home from work it dawned on us that we wanted to be more than friends, however the next day I was awkward and she was distant. Eventually we hung out and admitted our feelings and began dating, neither of us particularly wanted a relationship but were happy to see where it wrnt. one day we went to the beach, we were so close and affectionate with eachother and we both ended up realising just how much we liked eachother. After however I became worried at how quickly that one date developed our relationship and I was worried she was ready for commitment now which I wasn't, so I wanted to take a step back, she clearly had the same opinion however she took 2 steps back. We haven't hung out for 4 weeks and at work she's very stressed and depressed and has even cried because of how rough it is atm, she also wasn't in the mood to hold hands or kiss anymore, however I'm the only person she's openly annoyed and cold with and she qt least makes an effort with other colleagues. She text me to go for a walk with her yesterday and she just began to rant at me, that my chatty nature and joking attitude at work when she's in a bad mood really piss her off, I've not been as much help in work, it's not that she hasn't wanted to hang out but she's kept distant because of her mental state and that she does still want to be doing things together. After her rant she slowly began to come around and for the first time in a month held my hand and was happy to kiss again and even talked about our next date. She's still quite distant but it's a start. There's no other guy, so I'm just curious why she's become so cold and had such an issue with everything about me once we became very couple like.

What did I do wrong and how do I fix it? Because I'm starting to really care?
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