If during a flight I was stuck with a used diabetic testing needle left between the seats, what are my options?

I am sorry this happened to you.  I would notify the airline immediately and make an incident report.  You should also have a screening test done immediately for any viruses which can be contracted through a dirty needle.  You need to do this as soon as possible, as sometimes it is important to start certain medications for certain conditions as close to exposure as possible.  I would consult with a personal injury attorney in your area to discuss your options with respect to an injury claim against the airline.  Good luck to you.