Can you confirm the shower head is the same?
The most likely reason is the pump is oxidising the water (adding air) so for every 10 seconds of flow you would get say 1L, your now getting 0.6L, hence it feels like mist.
What is the model of your pump, you may find its just air in the pipe, once I know the model of pump I can advise if this is by design or something which will resolve itself.
Hi Ian, do you require any further assistance?
Hi Ian, in that case so I may resolve the case at this end please can you rate the service received using the star rating on this page.