The above post hit it on the nail: think about the ROI.
Public Health as a general field isn't known for its high compensation level. And, coming out with a MPH, yet without some type of work experience, will probably qualify you only for entry-level to low management type positions.
With 150K in federal student loans, you would approximate monthly payments of 1,000+ if using the standard 10 year repayment plan. With interest, this would likely total over 200K.
If you feel it's the right thing to do, go for the masters. Otherwise, perhaps get some work experience first, contribute as much as you can towards the undergraduate loans, and enroll in graduate education at a later date. The extra work experience can never hurt.