How do I make the case to regain first authorship? I am a senior Phd student in a very small lab consisting of me, my PI, and one other graduate student (one year under me). We began at an academic institution working on this paper related to my dissertation. I was always first author on this paper, and presented it to my committee as such. A former post doc who "contributed intellectually" but did minimal actual work for the final paper is second author, and the other grad student was third author. The other graduate student contributed significantly more work for this paper than the postdoc who left the lab due to lack of funds, so she asked if she could be moved to second author. 

In a STUNNING move, my PI made her co-first author and kept the post-doc at second. I found this unfair, and when I went to talk to her, she revealed that she had promised her post-doc high authorship on the paper before she left, and also felt that the other graduate student contributed enough to be co-first. I had to no choice but to let it go.

The paper was rejected three times in its current form. The last journal told us if we added more patient numbers, they would reconsider it with the same reviewers. As a result, my PI added 200 more cases (we only started out with about 150 cases) from a cohort that I managed myself, processed, phenotyped, genotyped, multiplexed, etc. I did all the statistical data analysis, redid all the figures (that I had already made to begin with) and integrated these extra cases into the manuscript. The other graduate student, nor the postdoc had anything to do with these additional cases. 

When I asked for my sole first authorship back, she blew up at me and told me it would demean the other graduate student by demoting her (which I felt is exactly what she did to me). We have an upcoming meeting and I need to know what my options are outside of her if she refuses to change the authorship list. Do I need to contact the department head and my committee about this? I told the other graduate student the situation and she is COMPLETELY fine with being second author, as she was never asking for first authorship to begin with as this paper is not at all related to her dissertation. She actually thinks its ridiculous that my PI is not seriously considering this request. I feel like she is trying to save face with her post-doc, and not giving me proper credit for my work.

Am I being unreasonable or do I have a case?
You could make the case that agreements about author order should be renegotiated after a major reworking of the paper. Because this is essentially a new paper, you might convince all parties to change the order. If you regularly correspond with the post doc, you might ask her if she would consider the new author list.

However, if she does not wish to change, you should accept this gracefully. Frankly, you have already missed the appropriate window for renegotiating author order. Author order should be determined before the work (or rework) is done. 
Trying to change the order after work is complete is inappropriate because if, for instance, the third author wanted to keep her place as second, she no longer has any opportunity to do so.

The correct course of action would have been to propose being reinstated as first author before you did the work on the 200 new cases. At this stage, your negotiating power would have been higher. In addition, anyone who wanted to maintain co-first author or second author position could have helped you with the work. It seems you have learned a hard lesson; I hope you can avoid letting it ruin your relationship with your PI.