During the past years of my PhD program in biology, my advisor has developed a habit of lending my service to other labs/groups for potential collaborating opportunities. In addition to helping fellow students in the lab with their projects, I've helped a handful of other labs/groups with various data analysis and statistical testing tasks that were totally unrelated to my PhD project; I rarely got anything in return other than practicing my relevant skills.
There is a lab in our department that I helped with data analysis in June. I've submitted my PhD thesis in August. This lab now came back to my advisor asking for a further work involving testing the correlation between the expressions of 200ish genes and different phenotype in 20ish RNA-seq datasets. My advisor has handed the job to me again.
It is not a small task to me, and since the submission of my PhD thesis my scholarship has stopped so I have to work to pay the bills. Also, I feel unappreciated because the other lab doesn't even know I am the person doing the job, and they will probably list my advisor as a co-author should there be a paper.
I'd like to know if I should turn down the request, even if it will potentially result in my advisor rejecting a recommendation letter? My advisor once jokingly mentioned that they are holding their recommendation letters hostage so I have to continue working for them (for free). I've done tons of unrelated and unpaid works for them and this project feels like the last straw.