What is up with this Scorpio Woman?

Could use some insight into this situation. So there is a scorp woman at my place of work that I've recently started working with on a project. Before we started working together I never had much interaction with her, but from the brief conversations I had with her she seemed slightly stand-offish. When I found out we were going to be working together I initially was not thrilled about it, but knew I should give her a chance because I did not know her. We started working together at the beginning of February, and things started off great. We got along really well. In the past weeks, I've noticed some odd changes in her behavior. She's been slacking off in her work and not meeting deadlines, which means I have finish her portion of the work along with my own. I've tried to talk with her about this, but all she does is gives me a list of excuses as to why she can't do her work. (Most of the time she claims she is too stressed out or tired to do it). Also she has started talking about other co-workers behind their backs. The other day she went on an hour and half rant about why she hates our supervisor. Our supervisor did get after her the other day (because she isn't doing her work) and she got all offended by this. I personally I am not comfortable talking about people behind their backs and I've tried to change the subject when she goes on a rant, but she always brings the conversation back to others. She has also started making subtle comments to me here and there suggesting that I am not good at my job.
Finally, she has started to be clingy to me. She always asks when I'm coming into work and when I'm leaving. She wants to work by me, and gets oddly upset if I have a meeting or somewhere else to be and she is not going. She's been asking where I hang out on weekends and if I want to meet up. I had the day off yesterday and she sent me five personal messages on Facebook asking what I was doing. As a Cap I don't do well with clingy people, and I'm trying to figure out what this behavior is all about. One of my best friends is also a Scorp, and she does't act this way.
Any advice on how to deal with this person?