No varicoceles, no hernia, no infection but still my testicles hurts. mild pain/discomfort. what could be the cause?

Testis pain: Most cases of pain in the testicle will resolve on their own. I would see your primary care doctor to make sure you don't have an infection first. A scrotal ultrasound may also be helpful. I would advise that if your symptoms persist longer than three months, you should probably see an urologist who is comfortable with treating scrotal content pain syndromes.