Do I have to pay back support after an agreement was established?

Wife cheats, moves out with child almost 3 yrs ago. Last year in mediation we agreed to a child support amount. Have paid for the last year. Just also paid a lump sum of half of all child care and medical expenses. I have my child more than her mother during the week and also pay for half of extracurricular activities. During our final mediation before divorce is granted ex wife is now seeking back support from the first year she left. Am I obligated to pay even though I have been paying for the last year. This was never brought up by her during any other mediation. Chiold support was not established by a judge for we were trying to be civil, but now it seems she is being spiteful.