Shower drain smell when water on

[elflord]

I have a classical elongated rectangular shower drain that is built into the floor (see the attached image) and it has bad eggy smell but only for the first few seconds after running the shower. Otherwise, there is no smell at all. There is water in the gulley trap so the sewage smell does not escape. I removed water from the trap to see if it is not dirty, but everything looked fine. It is only when i start the shower that the drain gives the eggy smell for the first few seconds. Any ideas why that might be? Cheers.