If your playnig with GWX installed, then yes you hit a mine, The team put the east coast mine barrier in the game. It has gaps in it at various points to allow ships friendly to Britian in and out of ports, unfortunantley the German high command didn't know where these gaps were.
Take a look in the GWX manual and you will see the diagram of the east coast barrier minefield. Hope this lifts any confusion.
