White thick milky discharge and back pain whatdoes it mean ?

Depends: White milky penile discharge, along with back pain can be due to several reasons, most commonly epididymitis, or prostatitis. However, other sexually transmitted disease cannot be excluded. If you have any abnormal genitourinary signs and symptoms, including: enlarged inguinal lymph nodes, pain on urination, penile discharge, genital rash or redness, urgent examination and tests are recommended.

May be "connected": See your Primary Care Physician. Your symptoms may be related to a arthritis disorder called "Reiter's Syndrome". Blood tests and a complete evaluation is indicated. There is effective treatment for this condition if diagnosed.