I am having pain in lower abdomen originating at belly button when i'm about to finish urinating. i already had uti once but that time i had backache.

Bladder infection : You likely have a urinary tract or bladder infection again. Women are prone to urinary tract infections because the tube that goes bladder (urethra) is short. Frequent sex can push bacteria up into the urethra and into the bladder. See a doctor and start treatment to prevent the infection from spreading to your kidneys. Urinate immediately after sex and drink lots of water to prevent a recurrence.