Interstate child custody disputes occur between the states (as opposed to intrastate disputes which occur entirely within one state). Interstate disputes are more and more common as people move and live across state borders. Unfortunately, these disputes dramatically increase the pressure on litigants. They require you to submit arguments to the court, even if you don't live there, and even if you live 1,000 miles away.
Interstate child custody disputes were once commonplace but no more. What would occur is that one spouse, unhappy with a particular result or looking for a more favorable court, would "shop" around to different jurisdiction. The spouse would file in multiple places to put extra stress and expense on the other spouse who had to face them.
Luckily, the most states adopted the Uniform Child Custody Jurisdiction and Enforcement Act (UCCJEA) which, mostly, prevents this behavior. The UCCJEA requires courts to pass a series of tests before it may assert jurisdiction over a child custody dispute. The goal is to prevent "forum shopping" by litigants. Unfortunately, you still must prepare and present arguments to contest jurisdiction, so the expense isn't eliminated.
If you believe it is possible that your ex-spouse may attempt to file for a child custody modification in another state, then you should contact a lawyer as soon as possible. You never want to leave these decisions to chance, even if you believe that there is no way the court will assert jurisdiction over your case. A lawyer can ensure that your arguments are heard and that the court rules in the correct direction.