There are some situations where doing a postbacc is recommended, particularly if you need to do "GPA repair" by taking a few more UG level courses to prove that you have the academic capability to succeed in medical school. Some postbaccs are just taking additional courses without getting any degree at the end of it, some are "Special Masters Programs" (SMPs) designed for pre-meds seeking to increase their chances at medical school admission and you get a certificate at the end, and Master's programs are their own degree with a required thesis to be written and defended after some additional graduate-level coursework. I'd definitely defer to some of the more experienced posters on here about the differences between programs and what makes each desirable for the prospective applicant, as I'm matriculating right out of UG.
You're definitely not in the realm of needing to consider a postbacc as a freshman with a 3.8 GPA, this is definitely a conversation that you should seek to have in a couple of years time. Also, why are you graduating early (or trying to, at least)?