50/50 legal and physical custody CS

I have been a very present parent! Always there for my child no matter what, as it should be. Well, I come to find out that the county has me registered as a non-custodial parent, and if anything it should be the other way around. Well, apparently my ex has told county services that I am no where to be found and she doesn't even know who I am. Weird considering we talk on a daily basis regarding our child. Well, it was apparently in effort to gain support services such as food stamps, medi-cal, relief money, and she didn't want me to pay CS on the money. So she lied regarding me. Well, financially she is better off then myself, and I have never sought county support because I didn't want it to interfere with hers. We have a divorce papers saying that she wouldnt seek child support, but I have a feeling that is admissable with changing circumstances. Anyways, I filed taxes this year and CS got my address and now have sent me papers for a Case Opening packet. I am not sure what to do with it. If I should fill it out and become liable for previous CS or ignore it and not be liable. If I fill it out then I will want to be able to fight any claim to CS that the county may feel I owe them. Although, I am afraid that she may then be hit with a large bill and I don't want that either. I have noticed that she is very careful and won't talk about this stuff via text. I am at a disadvantage, because she has done all paperwork for school registration, doctor, dentist, afterschool. She has always insisted and just asked me for money which I always gave. I just don't know what to do, feels like I am going to lose no matter what I do. Any suggestions?