Say nothing! You have a right to not incriminate yourself. You have a right to refuse to testify before the grand jury. Since you both live together, they are probably exploring whether to charge you as a party to the possession. However, there seems to be a question of custody and control of the drugs. I would need to know more information about your access to your b/f's safe. You may have a defense to any charges leveled against you. You should talk to a lawyer before you do anything further. You may call me if you wish.
You have a right to remain silent. Use that right. You may need to retain counsel who can be present while the police are speaking with you. Absolutely under no circumstances should you speak to an officer without speaking to an attorney first.
Answering this question does not create an attorney-client relationship. Legal advice has not been given. Also, this question and answer is posted on a public forum and therefore any attorney-privilege is waived.
Any suspect who is called before a grand jury should have a reputable criminal defense lawyer assisting them. Unfortunately for defendants, the defense lawyer is required by law to wait outside the grand jury room. However, the person who is the witness can excuse themself to ask the lawyer whether the question is one that should be answered or met with pleading the 5th.
So, that's some general info. You need to get a lawyer.
Answers on Avvo are for general information purposes only and should not be relied upon as legal advice. No attorney / client relationship is created by providing this answer. For specific advice about your situation, you should consult a competent attorney of your choosing.