Whitney Purvis has double the trouble with the authorities -- not only was the "16 and Pregnant" star picked up by local police, but the feds also arrested her and are now investigating the case ... TMZ has learned.

Jae W. Chung, Acting Special Agent in Charge of the federal Drug Enforcement Administration, tells TMZ ... Whitney was busted Monday by Atlanta-based DEA agents and officers from the Rome Police Department in Georgia.

As we first reported ...Whitney was hit with state charges of involuntary manslaughter and drug offenses in connection with the fentanyl death of John Mark Davis.

Chung says the investigation is ongoing and part of a broader crackdown on fentanyl-related deaths. He notes that nearly 70% of all drug fatalities in the U.S. currently involve synthetic opioids like fentanyl.

Rome PD officials say Harris died from a toxic overdose after Whitney allegedly distributed a powerful combo of fentanyl and xylazine, commonly known on the street as "Tranq."

Making matters worse, Whitney's legal troubles come just over a month after her teenage son, Weston, passed away from natural causes.