The doctor was right, you were never exposed to HIV from oral sex. You never required a test because your exposure was non existent, therefore you will always get a negative test, irrespective of the time passed
Thank you guys for your feedback. However I've seen in some other websites such as AIDSVANCOUVER Canada that Receiving oral sex is considered a negligible risk but not ZERO RISK. Still they claim that there is no documented cases which has been infected by this way. Thanks again