Confused about her.?

Let's see... I've dating my girlfriend for about a year and a half and for the longest time we've been extremely close. In fact we've been friends for about 5 years, so our relationship was built upon a friendship (which i've heard is a great thing). But maybe for the last... month or so? She's been easily upset by every little thing, barely smiles and I don't seem to be able to comfort anymore. I'll even give an example of how weird it has become: one night I mentioned maybe wanting to do something romantic (for no reason, just felt like spicing up the relationship a little) and she got scared like we were going skydiving and that I was some alien for wanting to do something completely random and having fun. We also argue quite a lot and she gets set off by the smallest things... I'm REALLY not sure what is going on, she only seems to care and show emotion after a fight. I'm ok with not talking as much and the relationship slowing down (less , so forth)... but what do I do?