good divorce lawyer question?or anyone who can help?

[mr. y]

how in the world can someone bring the other half back to court for more c/s .when an agreement was made and the judge finalized things.i.e i pay a certain amount of c/s.i live with my brother.then i get a better paying job so i can get my own place.now the ex finds out and brings me back to court for more money.yet ive signed a lease.knowing i now have my own place and bills can the judge make me pay more,put me in a position where i will end up breaking a contract/lease.to pay her.in which that would force me into pos getting sued?what do i do about this situation?how far can this go,i mean there has to be a point where i know i can safely get on with my life with this type of situation,right,wrong?