UNBELIEVABLE!:"He Left Us For Years!" Adrian Peterson Baby Mama Said.

Tmz

Adrian Peterson baby mama #2 Erica Syion left out one HUGE detail when she said the NFL star has taken good financial care of his kids -- back in April, TMZ has learned, she sued his ass for $52,032 in unpaid child support and other child-related bills. Erica --who has a 4-year-old son with Adrian -- went on "TMZ Live" Wednesday, claiming AP dutifully pays his child support on time.

But that wasn't the case earlier this year.

We've learned Erica filed court docs, claiming Peterson failed to pay child support for three straight years, and also failed to reimburse her for their son's travel expenses and school tuition -- a bill that totaled $52,032.

There must have been some bad blood after that -- because Erica filed additional docs in July, asking the court to order Peterson not to trash talk her in front of their son, and not to hide their son from her.

But it was all resolved in September -- according to court docs, the two reached a settlement ... which included a bump in child support to $2,500 a month. Erica tells us, Adrian paid all his back child support as well as the other bills.

But there's a catch -- according to the docs, AP plans to stop paying school tuition for their son by next year. Interesting.Calls to Adrian's camp weren't returned.