Blister on penis

Hi, just over month ago I noticed a greyish blister on the head of my penis. It started off painless but gradually started feeling pain on my penis and all over my body. I went to the doctor and was told that it was probably a viral infection and was prescribed antiviral and fungal creams. I got a test for Herpes which was negative. The blister had since dried out, but I still feel pain down my legs, anus, urethra and back. Does anyone know what this possibly might be and if it could be serious? I am a bit concerned because its taking so long to get better.

Thanks