Does the pump switch off electrically the same time as you turn off shower? Or do you switch the pump off manually, it sounds to me that you are turning off the shower and the pump is running on building up pressure, if so then you will have to make sure that you always switch off the pump first and also turn the shower on before you switch pump on.
If the pump is new it should have a built in electric sensor which detects that you are running the shower and then the pump comes on and goes off automatically.
I suggest you get this checked a. s. a. poss.
Cheers Hard@it