I clean my bathrooms very regular as I am very particular about odors as well as stains. However, I clean the toilet and shower and still there is a bad smell in my small bathroom. I suspect it is coming from the shower drain. It that possible?

You can verify if its coming from there by placing some plastic wrap over it and sitting a bottle of shampoo or something round over it. If it was coming from there the odor will disipate. If its not that... the odor will remain just as strong hours later.

I'd also look at the sink drain... particularly if it's a metal one... you can get corrosion that lets sewer gas in... but doesn't leak water.

It is even more likely your toilet has a bad wax ring which will allow sewer gas (methane) to seep into the room. You can test for this by packing putty or modeling clay in the crack between the floor and the toilet base. If tyhis seals out the odor then it was a bad ring which can be replaced for about $3

Are there any gurgles or bubbles when you flush or drain? Back to you, Tom