There are a variety of conditions that can cause the symptoms. First, there can be physiological conditions - such as testosterone deficiency, concurrent diseases (i.e. diabestes), or vascular problems. You note a normal testosterone level. You may want to consider imaging studies (i.e. an ultrasound) to ensure that there is appropriate blood flow to the penis.
If the anatomy is normal, then you may want to consider non-anatomical conditions (i.e. depression, stress, anxiety). Addressing these issues may be of benefit in improving your symptoms.
Regarding the testicle and lower back pain, you may want to consider an evaluation for prostatitis. A digital rectal exam by a urologist as well as a urinalysis would be a reasonable start.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Thanks,
Kevin, M.D.