I went to dr for my pain in left testicle she did an exam couldn't find any lumps or any out of the ordinary. ...She saud an ultrasound would be helpful. ..What else could I have?

Varicocoele. The main things to rule out are a testicular mass or torsion (where the tetsicle twists around on its blood vessels). Both of these are emergencies & it sounds like your doc already ruled them out. A common cause of dull testicle pain (espcially left side) is a varicocoele which is like varicose veins aournd the teste. An ultrasound can pick this up. Alos need make sure you odnt have a hernia.
See below. You could have epidydimitis which is inflammation of the ducts that transport sperm to the outside. This is very common. If the pain is very severe and there is swelling of the scrotal sac, you could have torsion of an appendage of the testis.