As you are worried I'd suggest trying to remove the plaster to see what's underneath where you hit. Hopefully it's nothing serious and you can just repair ths plaster.

Is it line with either a wall light or socket? Could just be a bit of conduit. Post a pic of the wall

If it was gas you would smell it. It has a very strong smell. If you do smell gas then it is an emergency and you can find the number to call online. Electrics are usually up to a light switch or a wall light. If it made a clang then possibly a water pipe (heating) but a small nail probably won’t cause any problem. I wonder if you could dig out some of the plaster as Sarah suggests so you can have a peek? Worth it just to reassure you.

If you suspect any damage to electrical wiring or gas pipes, and you know how to safely do it, consider turning off the electricity or gas at the main supply until a professional can check it.

It might be a good idea to have a professional (like an electrician or a plumber) come and inspect the area. They can use tools like a stud finder or an inspection camera to see behind the walls and ensure everything is safe. If you need some bath bomb for your bath check on wholesale bath bomb