A Doppler ultrasound is a noninvasive test that can be used to estimate the blood flow through your blood vessels by bouncing high-frequency sound waves (ultrasound) off circulating red blood cells. A regular ultrasound uses sound waves to produce images, but can't show blood flow. You need to do colour doppler to check your veins.
I'd tell this to you loud and clear.
Masturbation DOES NOT Cause any harm to the body or your penis or sexual organs or your mind and is a completely normal practice.
It won't harm you now nor ever in future.
Your lower back pain could be due to something else or may be psychosomatic.
Take a chill pill and have a happy sexual life!