A loyal former assistant to state Sen. John Sampson testified Wednesday at his obstruction-of-justice trial — and faced tough questions from the judge about her honesty, and from prosecutors about contradictory statements she made to officials.

During a morning turn on the witness stand, Celeste Knight endured a grilling by an assistant US attorney for apparently changing her story on how typical it was for Sampson to ask her to use her own cellphone to conduct some of his shadier business.

In 2012, Knight had told the FBI that it was unusual for Sampson to do so. But in court Wednesday, as she was confronted by multiple wiretap recordings to the contrary, Knight changed her story.

“Hey, do me a favor; do it on your cellphone and do it on your own time,” Sampson instructed Knight in one recording played for ­jurors.