Are you getting the burning after masturbating? If so you might be rubbing to hard or you might be getting lubricants inside your urethra. Also it could be due to not drinking enough water due to lubrication of the insides of your urethra. However STDs might take some time to develop and show symptoms or show up in STD test via Urine & Blood. If you feel you are infected go to another doctors office or hospital and have them test you.
Get back to your doctor and ask for a swab to be taken to check for fungal infections.
As a home remedy you can put natural set yogurt onto your cleaned and gently dried penis. This is only a temporary measure and you can use it as long as you need to, but you still need to get a swab done to be sent for analysis.
If it is a fungal infection, you will be prescribed with the appropriate antifungal cream or capsules to take by mouth.
Don't use normal soap, shower or bath gels, these will make the irritation worse. Use non perfumed soaps and those that have a pH level like our skin, baby washes should be fine. Don't wear manmade fabric underpants, wear cotton ones and don't wear tight clothing, this causes sweating which encourages fungal growth. fungus loves warm moist places.
Use latex free condoms.