Need help

Why does sexual activity trigger flare up with uti? I go toilet after doing anything intimate and still get a uti. I’ve had uti’s on and off for about 18 months now. Never doing anything sexual while I have symptoms or on strong antibiotics. I am on a low does of antibiotics and have been for 9 months now but still get uti’s. Also take d-mannose. It’s not even from actual sex. It’s mainly from fingers for some weird reason. And yes they are clean etc. I get positive uti results by urologist and they still can’t find the cause so I’m wondering how to stop because at this point I feel like I’m going to suffer with them forever and can’t live my life properly