Thank you very much for the input and advice. I will post a more detailed explanation and the options I was suggested by other PIs, including chair of department and postdoc advisor.
4 years back when I took the cKO mouse project I took it from the hands of a former postdoc who spent 6 years trying to make it, but failed. He left the lab without a first author publication. Most of the senior postdocs in the lab advised me against talking on such a risky project. My PI also stated that it is a high risk project, but he also stated that once the mouse is ready I will have a very good paper referred by many other good papers. Like I said, it was a pretty cool cKO, so I said yes.
At the time the mouse was ready my PI asked me to start 6 other colonies by mating the cKO mouse with other mice (to delete protein X in different tissues). My project is based on one of these mice. I gave my labmate the initial cKO mouse and she deleted protein X in cells collected from this mouse. So, while I was creating the other colonies she got a head start on the project, leading to a faster paper than mine.
Like I said, after her paper was submitted without my reading it first, I took advice from 3 other PIs. All of them said mostly the same thing:
1. I have VERY solid grounds to ask for first co-authorship, but be prepared for the lab mate to say no.
2. I can ask for the paragraph with the mouse description to be taken out of her paper. Since her paper will most likely be published before mine, she will still get a lot of credit for the mouse, but at least everybody who takes time to track down the mouse design will give me the credit (for a job interview I can also prove this was my work)
3. If all fails, I can go to the editorial board of the journal and make a bad case, and like everybody said, ruin my reputation; and most probably still lose in the end.
In the end after a long conversation with my PI he realized it was a big mistake not to be given credit for the mouse and asked me what I want. I chose solution #2 (take out the paragraph with the mouse description) since I do not want any more confrontation with the lab mate.
I am working hard now to get my paper out as soon as possible. Hopefully it will go in a much higher journal than my lab mate's, so this will mean more citations also.
I learned my lesson: do not share anything before you are sure you will get the credit you plan on getting (best is to get this in writing).
Again, thank you everybody for your input.