I changed bathroom light fitting and light in bedrooms gone but still light in bathroom. Any idea's?

I have seen this before, The black wire is not where need to be, I'm sure is connected with the red wire and that way when you close the light is on, or you have connected the red wire to the switch were is (com) point The best is to ask an expert NIC and to do a site visit

You have not connected something up properly. Rather than fiddling with it unsafely get a sparks to come and rectify it, it’s not a big job about an hours work.

bedroom light is probably looped from the bathroom light and a wrong connection or 2 made when you swapped fittings

It sounds to me like the wire going from the bathroom light to the bedroom light is a supply wire that might not be on a ring. therefore check the connections inside the bathroom light fitting to make sure all the wires are correctly installed. That or has the bulb in the bedroom blown? sounds silly but coincidences can happen. Hope this helps

First check power to bedroom light then check wiring In bathroom possible it on a loop and a wire is not connected right

Check loop connection for the switch red on black and black on red, honestly you need an qualified electrician to do the job, being handy man does not solve the issue