Why do i sometimes get backache after masturbation?

Strain/congestion: Depending on your method, your technique, and all the processes involved in your activities, you may be tensing your back muscles more than you know, arching your back, and generally straining muscles you do not commonly use. You may also have pelvic congestion that could cause some referred pain to the back and be confused with back pain. Doubt this is all of significance.