I masturbate 2 times a week but sometimes i have lower back pain next day after masturbation. why is it ? should i be worried ?

See below: Doubt if you have anything to be worried about. Lower back pain likely either related to a muscular strain or "referred" pain from your prostate related to some non-specific, inflammation or irritation. Both possibilities generally are self-limiting and require no specific treatment but if symptoms persist or worsen, recommend proper urological evaluation for reassurance and treatment options.