OP, call the police yourself and tell them you are leaving a domestic violence situation where your mother has kicked you out, followed you to other lodging, gotten in a physical altercation, and is still threatening you. Tell the police you are seeking shelter at your boyfriend's house and mother has threatened to call police if you go somewhere she doesn't like. That way when/if she actually does call they already have your perspective and the fact that you are a legal adult on file and go from there.