I have physical custody and mother has been helping out by watching child while I start new job its been 6 months and now she filed support and I have to pay how is this possible

Support is often based on the amount of time the child spends with each parent. So while not normal, I have seen it done.

Has there been a Court hearing?

She is probably applying for a modification of the custody order since the child is spending more time with her. As part of the modification, she is asking for support. Sounds very possible to me.

When the issue comes before the court and if the child is living with you then prove to the courts that you have primary physical custody. There may or may not be child support as ruled by the courts. She may be the one paying it. We can't answer without knowing the whole situation. But in general terms the one with primary custody usually doeant have to pay support. The NCP (non custodial parent) is the one who pays.