EXCLUSIVE

3:29 PM PT -- A rep for Swearinger tells TMZ Sports ... "This incident was an unfortunate misunderstanding, and we are working on resolving the issue."

The rep adds, "DJ is just focused right now on his off-season training and looking forward to the upcoming season."

Houston Texans safety D.J. Swearinger is under investigation in Texas ... after the 23-year-old allegedly stole his own truck from a customizer without paying for $20k in upgrades ... TMZ Sports has learned.

It's a bizarre story, but here's how it breaks down:

According to a police report filed in Houston, Swearinger took his 2013 F250 truck to Espi Motors to get his ride pimped out like a boss -- matte black paint with a Batman logo, speakers, new wheels, custom grill, train horn, etc.

But according to Espi, when he came to pick up the whip, he got sticker shock ... and drove it off the lot WITHOUT paying for the work.

Espi tried to track down Swearinger to get the cash, but D.J. went M.I.A. so Espi filed a police report.

Law enforcement sources tell us cops are investigating the situation.