My wife will not let me get a vasectomy?

When I married my wife 2 years ago and I told her BEFORE WE GOT MARRIED that i did not want to become a father and she also said she would NEVER want any children. So about a week ago I caught her talking to her sister about her stopping talking the pill because she wanted children. when I told her i wanted to get a vasectomy she went into a tantrum and start to scream. She said that we could not afford an operation at this time(which i do not understand because we are both nurses). I just think she is trying to trick me into fathering a baby.So clearly she wanted children the whole time and just lied to me. I do not understand because it is my body, she can not stop me. to It would be like me telling her to get an abortion and me getting mad. please help understand my wife. what should i do ?