Jennifer Rodriguez claimed this week in court documents, obtained by TMZ Sports, that back in September 2021 -- following a yearslong, non-exclusive relationship -- she became pregnant with Tatis Jr.'s child.

She said in the docs that, initially, she had hoped the two would work together to co-parent their kid ... but following a paternity test that confirmed Tatis Jr. was the father -- the baseball player and those close to him began treating the "impending birth like a business deal."

She said all of their conversations prior to their daughter's June 2022 birth date led her to sign an NDA ... and an ensuing private child support agreement that she says she had to negotiate "on my own" while nine months pregnant.

As part of the pact, Tatis Jr. -- who inked a massive, 13-year, $240 million contract with the Padres in 2021 -- agreed to pay Rodriguez $20,000 a month.

While she said Tatis Jr. has since fulfilled that obligation ... she claimed in the docs that following their daughter's birth, he's been largely absent from their kid's life.

Rodriguez said despite she and him continuing their romantic relationship through May 2023 -- Tatis Jr. has only seen his daughter on five separate occasions ... for a total of four hours.

And, she says to make matters worse, when she recently approached him and his close circle about paying more in child support when she learned that his yearly salary would increase from $5,000,000 in 2022 to a whopping $36,000,000 in 2029 -- she received pushback.

She's now asking a judge to bump up what Tatis Jr. has to pay in child support payments.

"I have come to understand how little [our daughter] has compared to the benefits that Fernando receives for being a visible MLB star and local San Diego celebrity," she said. "I have also come to understand how much it will cost to support a child to live at Fernando's station in life."

In the docs, she claimed 25-year-old Tatis Jr. resides in a large home, drives fancy cars, and "that in San Diego, if not all over the country, doors open for him in every facet of his life."

"Our daughter is entitled to share in that lifestyle," she said.

Additionally, Rodriguez is asking for Tatis Jr. to pay her attorneys fees -- which she says total around $250,000.

A court hearing on the matter has been slated for December.

For Tatis Jr.'s part, his lawyer, Michael Strauss, said in a statement to us on Friday, "Ms. Rodriguez’s decision to push out a false narrative about Fernando and the Tatís family is nothing new."