'Southern Hospitality's Grace Lilly Arrest Was After Lab Identified Klonopin Pills

Grace Lilly's latest arrest last week apparently didn't come out of nowhere ... it seems to have been triggered by new lab results tied to her previous run-in with cops, TMZ has learned.

According to an incident report, obtained by TMZ, cops say they tested the pills police found in the "Southern Hospitality" star's purse during her December arrest in Charleston, SC. Authorities say the lab results showed the 12 white pills contained clonazepam -- which is often sold under the brand name Klonopin -- a controlled substance commonly prescribed for anxiety and seizure disorders.

It's illegal to possess the medication without a valid prescription ... and the report says an officer drafted an affidavit and a judge signed a warrant, which appears to be what ultimately led to Grace's arrest on March 10.

As we first reported ... the Bravo personality has now been arrested twice in roughly three months. Back in December, she was pulled over for an alleged traffic violation ... and during the stop, officers say they discovered what they initially described as "happy pills" inside her purse, leading to her first arrest.