Thank you for choosing DoctorSpring.
Discomfort in urethra is one of the most common non specific and possibly psychosomatic complaint. It could be due to a minor tissue injury also. Since you have the STD tests negative you can just wait for a couple of weeks to see whether it resolves on its own. Try to refrain from masturbation and sex for 3 days so that the mucosa can heal if there is any aberration. Stay well hydrated.
If the symptoms persists beyond 10 day I recommend a repeat STI testing
Hope this helps
Feel free to ask followups