A rotten smell can indicate a demonic being, especially if you smell it throughout your house as if it moves. Granted, the smell could be the result of many logical things, like a sewage leak, but an animal like a rodent could of gotten between the walls and died, or you dropped some food a while ago. Could also be mold if you threw a wet towel in your closet and forgot about it, or mildew especially if you just moved something out of storage in the basement. I would clean your room or perhaps house [if its in multiple areas] thoroughly. If you live in an apartment or a suburban area ask your neighbours about any smells or septic issues.
If it is a demon, there would be many more signs like sounds, the feeling of being watched, injuries, and/or objects being broken or go missing.