Hmmm... could be complicated. Real questions are, how much do you like this girl? and how much do you respect her?
If you just like her, think she'd make a good booty call, etc. then you can string this along for a while without much to worry about.Since it's SHE who confessed love first, she's already made a monumental commitment in her mind (well for a typical female anyway, and especially based on what you said about her history) and so is likely not all that willing to give you up easy. Heck, I've known enough girls who love the slime and stay by their side regardless of how they're treated.
Now, for the respect angle. If you respect her, and want this relationship to have some kind of a serious future, it's best to tell her to cool it. How you'd go about doing that I can't say, you know this girl best of anyone here. Since you're writing here and apparently feeling conflicted about her confession of love, you should explain this to her now, so she dosen't start making long term plans when you're still thinking about looking elsewhere.
The latter approach also has the added bonus of flushing her out if she's a realtionship psycho and regards you two as a couple already. If she acts like this is the worst thing in the world that's happened to her, and generally goes off the deep end, that's a pretty clear sign that the relationship was not meant to be. (yet)