Do i have to pay child support?

I think my wife went for child support, but i dont see how i should have to pay considering we have a court order ruling saying that we have joint physical and custodial care. i see him half the week, she sees him the other half of the week. im supporting him half the time, and so is she. plus i make about the same that she makes. not to mention we own a house together and the mortgage is all on me. once im done paying the mortgage i have about 100 dollars left for the month to live off of. how in the world would any judge say i would have to pay child support to her when we make the same we have shared custody and i have him half the time where i support him during that time. why is it always umed the woman automatically gets support, why cant she pay. she lives rent free while i keep going farther and farther into debt.