What does it mean when she says "I need some space" ?

I was "talking" to my ex gf for a couple years,(the time we were separated) we were both with other people but there was still alot of love thats why we kept in touch, we broke up with the people we were with and started dating again for a few monthes now, in the beginning it was beautiful, she called and texted me every chance she had and really showed me alot of love, we even seriously talked about getting married and starting a family. For the last couple weeks for no reason shes been really distant with me, she hardly calles me and sometimes doesnt reply to my texts for hours, and wen she does its short vague replys, naturally I asked her if everything was alright, she wouldn't really tell me, she would act like nothings wrong. I kno somethings wrong but I dont kno if it has something to do with me or something personal shes dealing with so I asked her to tell me what was going on, I told her that if she needed someone to talk to that im here for her and she has my support, and all she said is that she needs some space. I dont kno what thats supposed to mean, im gonna give her all the space she needs but im kinda lost and not sure if I should see this as a sign that its not working out or that shes going threw something personal.. I never smothered her in any way, I always gave her space, she would smother me but I loved her attention, we never even got into any fights.. I love this girl and I wanna do the right thing, What should I do? Please help a homie out...