What if I am not the right fit in a certain lab as a postdoc?
I presume that your postdoc advisor is the one that was directly responsible for your hiring. In such cases, she is presumably well aware of your skill set and former training, Presuming that she has not intentionally set you up to fail (a reasonable assumption, given her investment of time and money in you, and the potential damage to her reputation and your career), you should look on this as an opportunity to demonstrate that your PhD is well-earned. After all, the goal of a PhD is to certify that you are capable of learning a new discipline and making a demonstrable contribution to the state of knowledge in the field. Being put into a project outside your "comfort zone" is a perfect way to accomplish this.