It's a social control thing. She doesn't want to feel like you've 'won' anything at her expense. Having you dangling on a string ready to pounce at any whim is exactly where she wants you; it panders to her ego. It's base immaturity.
She's a manipulative, immature whore. Do yourself a favour and go through the courts. Keep civil so you can still see your son, but as for her, you're done. Do not ever allow her to manipulate you again, in any way. Never go running back to her and do not ever be intimate again. I don't care if you're ready to pop and she's got a hand down there, you walk away.
If what you've said is true, she's a vindictive, spiteful and callous woman. She shouldn't have been allowed to breed, let alone string hearts along for the ride. No one deserves her. She's not better than you. I know she's the son of your child, but she's evidently not worth a second's thought. She is exactly one of the things that is wrong with the world. Open your eyes. Don't allow her to raise your child with lies and untruths about your character. It isn't a fashion accessory, it's a child. Be a good parent and do what you know needs to be done.