Another cause is using soap when masterbating in shower.
A test for STD declares whether or not you have STD. There are several unrinary-tract infections that cause painful urination. Women are more apt than men to have bladder infections. I wonder when your were tested for STD, that your urine was also tested? You may consider having an urinalysis run to verify whether your urinary tract is clear. In Texas, where I live, major supermarket chains now have small clinics inside their stores. One can leave a blood or urine sample that will be sent to a testing lab. The estimated charge is about $40-$50.
Harrald
Did your Dr check for a urinary tract infection? Remar