Technically you don't owe any child support at this point.
Monies handed over without a court order can be considered gifts, but honestly in this situation if Mom was awarded back support there's a good chance you would receive credit for what you've paid so far.
If Mom does file eventually - or if you come to an agreement in court - the obligation generally doesn't go back much further than the date the petition is filed. So you appear to be in pretty good standing as it is.
I'm not sure that I'd stop payments at this point, since you've been doing it consistently.

