OP, the best way to determine if you can claim legal residency in Massachusetts is to talk to a Massachusetts lawyer or the admissions office at the UMASS medical school. Massachusetts does have rigorous residency standards. However, I'm not sure that your parents' move to California means you are a California resident or have no residency status. You are over 21 and emancipated. If you have filed Massachusetts tax returns all these years and move back now to Massachusetts, you might still be a Massachusetts resident.
If you look at the GPA and MCAT tables on the AAMC website you will see that California residency is pretty lousy for med school applicants and Massachusetts residency isn't much better. If you do end up without a state of residence, move to Ohio or Michigan. These two states are flooded with state supported medical schools and offer you the best chance at admission. You will need to lose that Boston accent, however.