My ex husband has been paying child support since our divorce. He just got married and stopped paying, is this legal?

It is not clear from the information you provide in your question but it sounds as if the child support was being paid voluntarily without a wage order.  Your ex-husband still has the obligation to support your children.  You have a couple of options in this situation, file a motion with the Court where your divorce was handled requesting that your ex-husband be held in contempt for failure to follow the ordered support.

At that time, you would request the back support you are ordered, request that a wage order be issued for future support, attorney’s fees, and costs.  Your divorce is final, but the court retains jurisdiction for matters involving custody and child support when you file your petition you may also want to consider whether your original order of support continues to be adequate for your children’s needs.

Increases in daycare costs, medical expenses, or your ex-husband’s income are just a few of the reasons why a child support order may need amending.  Your other option for pursing child support would be to contact the office for child support enforcement in your area.  They can also assist you in pursing you child support, but be advised this may take longer because of the backlog of cases.

You should consult a lawyer with experience in family law.  They can advise you in detail about the options available in your jurisdiction to pursue child support obligations due you from your ex-husband.  There have been changes in the law in the last few years making it easier to pursue child support and imposing penalties on deadbeat parents.