Hey.
If this is an electric shower (with pump + heater inside the unit on the wall), then it is strange that it only leaks when it is turned on. It could indicate a blockage inside the unit - or maybe the pressure of the incoming water is too high. To check blockage you can remove the front of the unit and check. But I would recommend that you get whoever fitted the shower to do this (you would need to turn off the trip switch to look at it, but then the test could involve turning the shower on with the trip switch reset and the cover off, and obviously this is dangerous so think safety and get a qualified person to check).
If it is a pumped shower (mixer on wall, separate pump in hot press or attic) then it sounds like it is a problem with the connection at the back of the mixer. Which may or may not involve removing tiles, depending on how the mixer is fitted. Depending on where your pump is, the problem could also be there - check for leaks here if your setup has an external pump.
To be honest, if you had the work done by somebody else, then they should fix this for you free of charge.