The arrow points in the wrong direction
The iPhone's compass may not be functioning properly.
To calibrate your compass, move away from any magnetic fields and hold your device in different directions.
You can turn it like a steering wheel, not like a screw driver. Between turning it in different directions, hold in still for a few seconds. After calibrating your phone, the arrow may start pointing to the correct direction.
You can also open the Compass app and verify the direction of true north.