I am a US citizen and student, and I would like to go to McGill for undergraduate. I understand that many US schools do not accept international students, but does the fact that I am a US citizen change this?
As an American citizen you will not be an international applicant to a US medical school.
Canadian medical schools are fully accredited by the LCME so undergraduate pre-med education in Canada is in line with American standards. In fact graduates of Canadian medical schools not even considered to be “foreign” medical school graduates when seeking employment in the US.
My older son graduated from McGill and had a friend there, a fellow American and a MA resident, who was accepted at BU, Tufts and UMass medical schools (rejected at Harvard Med, can’t win them all).
12% of McGill’s undergrad enrolment is American (and that may increase next year 
) so you will not be alone.