FAFSA doesn’t give any financial aid; it is just an application for federal student aid. It determines your eligibility for Pell grants (which per your other threads you will be eligible for) and subsidized student loans.
Please be aware that Pell grants and subsidized federal student loans have caps on the amount of credits that they can be used for. If you use your Pell and sub loan eligibility to pay for your nursing training, you will have less federal FA available to pay for your pre-med coursework and bachelors degree later on.
This is a decision you need to consider carefully. The need to support yourself near term vs achieving your long term goals. There are arguments to be made for both sides, but only you know your own particular situation and can decide what is best for you.