I didn't find "nag" in your sentences. But let's look at these:
My wife gave a birth to our baby by Cesarean section.
- great -
Because of the pain or not, she has been cared our son so much.
- this is not clear. Try again please. This sentence doesn't make sense. For one thing, I'm sure if you are saying she didn't care ABOUT your son, or FOR your son. It is also possible that your sentence might start better with "Whether it was because of the pain or not, ...." -
Even she bought very expensice stroller that is extravagant.
- "bought an extravagantly expensive (spelling) stroller" might be better here.
At the age of 5 of my son, he got pocks all over his body.
- don't use the "my son, he" construction. Just say, "my son got pox" (not pocks, by the way)
- and it's not age of 5 of my son, just "at the age of 5, my son got..."
Then my wife held him captive for a week in order not anyboby to get the pocks.
- "Then my wife kept him in quarantine for a week so that nobody would get the pox."
Now it's been 20 years since she gave a birth.
- since she gave birth (not a birth).
I've never thought of alimony in my life because I've loved my wife
- either "because I've always loved my wife" or "because I love my wife" - and I would probably add "unconditionally" to the end of the sentence, as alimony is a form of condition.