@Sejarah, that’s definitely understandable. Most 6 year programs that are now left (there were 3 for a while in recent years, although after this year, I believe UMKC will probably be the only 6 year program), don’t really allow a very well-rounded undergraduate education, in terms of coursework, outside of the sciences, due to the accelerated timeframe.
Are you taking loans on room and board (living expenses) as well, or just tuition? Financial aid, normally will cover both if needed since it’s part of COA (cost of attendance). You can use this student loan calculator (there are others as well): http://www.finaid.org/calculators/loanpayments.phtml, to calculate what that 40K (or more if you add room & board) will look like with interest. Tutoring during the summer of 2nd year undergrad and on the side during your self-selected 3rd year undergrad, probably will not make a huge dent in terms of your extra debt burden for that year (realize student loans have interest - about 6.8% but this varies by lender).
If you finish the 6 years and graduate with your Bachelor and MD degrees, your pursuit of a Masters in History will probably be less likely, not more likely, for several reasons: 1) in the fall of your last year of medical school, you apply and interview for the residency match in which you find out in the spring where you got into for internship and/or residency, 2) residency program directors tend to like recruiting applicants who are fresh U.S. seniors (meaning students who are in their very last year of medical school, and haven’t graduated with their MD yet). You probably would not be able to do the Masters until after residency, if you decide to stop there, or after your fellowship, where you could then go back. Now, whether you’d actually want to go back at that point, is a whole another story.
So for your stated purposes of exploring an undergraduate curriculum outside the one already set for you by your Bachelor/MD program, now would be the best time to do that.