Sources with direct knowledge tell TMZ ... the singer has missed his last 2 months of child support payments to his ex, Drea Kelly. He's required to pay $20,833 a month to his ex.

As you know, Kelly was jailed in early March after racking up a $161,633 delinquency ... this after he was arrested on sexual abuse charges.

Kelly was released after he cleared the debt after someone covered his tab. After getting out, Kelly went back to court and asked a judge to lower the $20k amount, claiming he could no longer afford it. The judge said he'd hear arguments but that won't happen until May 8. In the meantime, he owes $20k+ a month and it's clear ... the judge means business.

We broke the story ... Kelly was paid $22k for a 1-hour nightclub performance in Illinois earlier this month -- which Drea was totally cool with ... asking the media to take it easy on him while he tries to make some money. Turns out, he didn't turn over the loot.