Confused about wiring connections for new bath light

Hi I have an older home (1950s). Am trying to install new bath vanity light(replacing old vanity bar w/4 small bulbs). There is a light switch controlling light . I did not take pic or make diagram of previous connections.I did install an old work electrical box as the other wires were just coming out of hole in wall. Currently, this is what I have: two cables (each has B copper wire and W copper wire.) Also there is a single W wire separate from the others. I'm wondering if that is a ground wire? I have connected all B wires (2 from wall and 1 from new fixture) and did same with the 2 W that were in cable and 1 from new fixture . I attached the fixture ground to single W wire. I can get all 3 bulbs in new fixture to come on, but are not controlled by light switch, and it trips the breaker each time. Thoughts, or do I need to call an electrician. I have been able to determine the one of B wires is hot and other is not. Is the cold B part of switch?