Your negative test does not conclusively rule out infection due to COVID-19, but means that it is more likely that your symptoms were due to a different infection, such as influenza or other bugs that cause coughs and colds. It is important that you do not pass on these infections either, and should continue to self-isolate until you are better.
If you subsequently become unwell again, you should self-isolate, and may need a further test.
If you have had a NEGATIVE test result but have never had any symptoms, there is no need to self-isolate. If you subsequently become unwell, you should self-isolate, and may need a further test.