It was most likely the father or someone he lives with (e.g. grandparent).
If someone else claimed your child inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming the child as appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you'll receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the child, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.
Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest. The custodial parent almost always wins. The non-custodial parent can only claim the child as a dependent if the custodial parent gives permission (on form 8332).
The IRS will not enforce domestic court orders. If the father doesn't like you claiming the dependent, his only remedy is to take you back to court. That's unlikely, since his ability to claim the child is based on payment of child support and he's $20K behind.
