Child Support

I have two children. My son is almost 20 years old and my daughter is 17. She is attending an early college program. When my son was 16, he came to live with me and I paid a very small amount of child support (compared to what I pay now) for my daughter’s insurance. Since my son lived with me and my daughter lived with their mother, the child support basically cancelled each other out. A few weeks ago, my daughter told me that their mother was pushing her to remain in high school a full extra year so that child support would continue. I sent an email to their mother which illustrated why I did not think this was a good idea for my daughter and also highlighted the new law that states child support stops when an early college student reaches their 18th birthday and completes four years of high school. Today, I received a letter from NC child support that states that I must continue to pay child support until my daughter reaches her 19th birthday because our original divorce order was completed in Indiana and the order was domesticated to NC in 2001. Indiana law requires child support payment until 19. However, since my son lived with me when he turned 18, my children’s mother immediately requested and was granted that her child support stop, which increased my child support that I paid to her by several hundred dollars per month. It is not right that she “paid” child support for my son only until he reached 18, but is requiring that I pay for my daughter until she is 19. What should I do?