From the tumblr postings it seems that:
Chelsea was very upset over a relationship that was broken off by an ex-boyfriend.She felt humiliated because he didn't break up in person but simply changed his relationship status on Facebook. I am trying to figure out who this person is, but there are hints in Chelsea's postings. Of course, this person may have nothing to do with her murder, but we don't know. From the various posts I am trying to describe the history of the relationship by trying to "connect the dots" between the various postings. Disclaimer: detalis of the description can be wrong. It is not always easy to figure out the meaning of a post, or who it is directed at.
The relationship was broken off before August 12, 2011.At some point, he got a (different) girl-friend. But at the same time (it seems), he also had a FWB relationship with Chelsea. She states that he cheated on his girlfriend a couple of days before September 10, 2011. It seems that he and Chelsea work at the same place (Olga's kitchen). At some point he does not talk to her at all. In early February of 2012, Chelsea asks him to choose between her and the (current) girlfriend. In May she states that she is no longer satisfied with the FWB relationship. In June 2012, he asks her not to send him text messages. On July 30, 2012 he is coming back from going somewhere (for a week or more). Gradually she seems to resign to the fact that they probably won't be together again.
I am bringing this up, because there might be a chance that this person was also at the party. The pussycat dolls song may be random, but perhaps she had a plan of seducing him, showing him that she's better. Note that the poison ivy character has a seductive side to her.
(There is also a movie called "poison ivy" where the title character (unrelated to batman) seduces the father of a girlfriend.)
Perhaps loosing the phone, staying later was a way to get to a situation where HE would have to drive her home. This is all speculation of course.
So I think it would be good to know who this person is in order to look more into this. Does anyone have any ideas? If the person was indeed a fellow worker at Olga's than this limits the possibilities considerably. I think that in the group picture where Chelsea is in her poison Ivy costume, there is a tall person in the back with a Chicago Bull cap who looks like her colleague JL (although I am not yet convinced that he is the love interest). There may have been other coworkers at the party as well.