Chad Ochocinco -- Ambushed with Child Support Lawsuit ... Chad Calls It Unfair (VIDEO + PHOTO GALLERY)

Chad Ochocinco was confronted by his ex's mother -- who demanded the NFL star pony up the $38k in child support he owes her daughter ... but Chad says the amount is just not fair.

Here's the backstory ... Chad was leaving his son's middle school graduation in California last month, when he was confronted by the grandmother of one of his other children and some of her friends.

The woman was trying to serve Chad with court documents -- in which he's accused of stiffing the mother of his 14-year-old son out of $38k in back child support.

TMZ Sports spoke with Chad ... who tells us he's not trying to screw anyone over -- he simply can't afford to make the high monthly payment that was established back when he had an NFL income.

"I haven't had an NFL paycheck since 2011," Chad says.

The woman says Chad was ordered to pay $6,000 per month in support ($72k per year) plus another $1,000 per month needs to be contributed to the child's college fund.

But the woman says Chad hasn't paid in full since Dec. 2015 and he now owes close to $40k.

Chad tells us he kept up with the payments for years after his NFL career was over -- but his financial situation has changed (he has more kids and no income) and he can't keep up.

"I have a nugget, but I have to make the cash last with all 6 kids for the next 18 years. We have to scale back."

Chad says he wants to work out a compromise with his ex out of court -- but if they can't, they have a date with a judge set for August 2nd.