I requested a DNA and I use the child support agency because they provided it for free. My ex, did not let me sign the birth certificate but she wanted the child support ... it does not matter if I signed the paper of not, I was gonna pay support because the child was mine.
If she files for full custody, you will pay more. I currently pay $585 and I only see my daughter on the weekends. Not sure why you cant prove that your child is not yours and then going to an attorney and make the CS stop, if the child is not yours. Forget about embarrassment ... society will not help you pay your CS, so why be concerned about embarrassment?
Good luck and remember if you managed to get a better paying job, she can file an overview and take more money based on the new income that you and her have(assuming that you she works and has another job).