Hello, im having sudden sharp and stabbing testicle pain. it has been going for a week now. im a healthy 18 years old. the doctor took an image to see if there is any torsion and he said everything is normal. what could this be?

Pain persists for one week sounds concern. At least, you already had the ultrasound, I assume, done so torsion was ruled out. Did the doctor do a thorough exam on the affected testicle? Do you feel anything when you palpate your scrotum and do you have any constitutional symptom such as fever or chill ...? Sometimes, pain at the genital area could be a referred pain from the upper urinary tract.