I have been having giddiness, headache on & off for 2 weeks. no cough, flu symptoms. my gp cannot diagnose anything. i am concern of serious illness?

Usually a good sign: As a neurologist used to seeing really nasty stuff- I'd say NOT being able to find anything after a couple of weeks is usually a good sign. Nevertheless, I understand your air of caution. Perhaps, you should keep track of ALL SYMPTOMS on a log. Sometimes, this leads to patterns that your GP can better recognize and organize into something treatable. Make sure you're not overusing OTC meds.