My childs mother never informed me that my daughter was getting a disability check?

My daughter was diagnosed with a slight form of autism about 2,3 years ago .me and her mother have been split up for years and have 50/50 custody of her and my son...we lived close to eachother and both handled daily responsibility..we slipt the earned income tax money every year for the kids...about 2 years ago my daughter started getting a disability check for 750 a month witch I just found out about because we decide to move in together and I happen to get the mail that day ...a year ago the kids lived with me for sixth months she was not working ,she quit her job and started school and was living off of that check,while I took care of the kids with my own money and she did nothing..we moved in together and i payed half the rent for 6 months while she live off the check and just spent the money I gave her...I feel that it was wrong and she owes me my money...that money should have been benifiting the both of us..mean while my car payent is late bills pillin up im spending my money on food cloth shoe and she is layin up all day waitn on that check .....so from now on im not payin anything and when tax season comes im keeping the whole thing..am I wrong