I am not positive if this is the same across all programs, but I believe that during your 3 years of academic and clinical work prior to your 4th-year externship, clinical rotations are mandatory and are unpaid.
This is in contrast to other degrees such as engineering where their "co-operatives" are paid, but are also not counted for college credit.
Most clinical rotations are mandatory probably because a certain number of supervised hours are needed before the 4th-year externship. This follows the trend of unpaid rotations during the 3rd year of medical school.
Some 4th year externships do offer a stipend to account for students still paying tuition during that 4th year. This is not a guaranteed, though.
I was able to have a part-time job during my first two years, which helped with grocery costs. I decided to quit during the 3rd year because I needed a flexible schedule to test research subjects for my Capstone.
I hope this is helpful information. Good luck!