Can someone explain to me how come my gf getting bladder infections after sex?

Yes: The female urethra is the "tube" that allows urine to pass from the bladder to the outside when one urinates. This opening is above the vaginal opening. This "tube" is short in length compared to a male. During intercourse bacteria can be introduced into it, and reach the bladder more easily because of its short length. Follow up with a physician for further recommendations.