Whats going on with my relationship?

So my GF and I have been together for around 18 months and things all of the sudden she hella changed. When we first met we were crazy about eachother. We met in traffic on the freeway in the morning on our way to work. I was merging onto the freeway and looked over to see if it was clear to get in and I saw her. I'm not sure why but we couldnt stop staring at eachother. I asked for her # and she said "maybe later in traffic". So a couple miles down the road we met up again but this time I kinda boxed her in and told her she I would move my car after she gave me her number ( we were both totally cracking up so I promise it wasnt creepy despite how it sounds). Anyway she gave me her number and things moved really fast from there. I thought I had been in love b4 but I was wrong, I felt (still feel) like a totally different person. Then we moved in together after about 8 months, and things were still going great for another 8 months. Then one day a couple months ago she just all of the sudden started acting really distant. It went from sex everyday for 16 months to her not even letting me put my arm around her when were laying down in bed. This didnt slowly happen, it was one day we were all good then the next she's acting like she cant stand to be around me. From the begining I knew this was too good to be true so I really made an effort to keep things from getting boring. I'd always find new things for us to do, new places to go etc. Every other relationship I've had the "honeymoon stage" would slowly go away but never to the point where I felt like I was hated. Ive asked her whats wrong and she says nothing. I've asked her if she still loves me and she says of course. I really feel like she's not being real. I asked her if she wanted me to move out and give her some space and she started crying and saying she couldnt picture her life without me, but something is wrong. I love her but 2 months of her making me feel like a ******** is starting to take its toll. I do everything for her, and she cant even be in the same room with me for longer then a few minutes. WTF?????????????????????????