I am sorry that you are in so much agony.
There are two possible reasons for this.
1. A urine infection. These are not uncommon after delivery. For this you will need to get a urine examination done and then some antibiotics like norfloxacin
after prescription usually help in providing quick relief. Along with this you must drink plenty of water to help flush any bacteria in urine.
2. The second possibility is that there may be a small tear around the vaginal opening or inside that hurts when urine touches it. For this you must see your post natal care provider, preferably your gynec and have yourself examined.
I hope I was able to help you.
Take care and get well soon.