Two things come to my mind:
-You might have varicose veins in your vagina. You could ask your midwife or doctor to examine you, or you or your partner can look big purple veins in your vaginal wall. For this you can try wearing support hose, getting exercise, and taking time to lie down with your feet elevated. There are also various herbal and homeopathic remedies you can look into.
-You may have a sudden increase in pressure on your pelvic floor (perineal) muscles, you can tone them with daily Kegels excercises
-you probably don't have a bladder infection, but it never hurts to check either... and are you sure you're not constipated?