I'm extremely worried I've got testicular cancer. Can I just get an ultrasound scan just to be sure?

Probably. If you are a man between the ages of 15 and 40 and feel a mass in the scrotum, you should have an ultrasound as soon as possibly as initiation of the work up for testicular cancer. Ultrasound is essentially side effect free, and early diagnosis is extremely important in this highly curably cancer.
Testicular cancer. Well you generally have to see a doctor who performs an exam and then orders the ultrasound. Oh just man up and go get it done. After all lance armstrong, the greatest of all bike racers, was able to do it the first time!
Go see your doctor. The classic sign is painless testicular mass. Please see your doctor because there are tests that can be done to be sure. Measurement of markers, like serum alphafetoprotein (afp), and beta-human Chorionic Gonadotropin (b-hcg), and chest x-ray (cxr), and ct scan of the abdomen and pelvis can determine the stage of the tumor. If treated correctly cure is seen in 95% of patients, so get treated.