A state trooper who was popping prescription painkillers to beat a career-threatening injury was arrested yesterday on charges he knocked off a Natick drug store's OxyContin supply at gunpoint the day after Christmas. Christopher Shields, 41, who's been on medical leave for more than a year, was picked up at an undisclosed health-care facility in Worcester where authorities said he checked in after getting bagged shoplifting soda, DVDs and magazines in Sudbury last Thursday. ``There are some personal problems going on here we hope to be able to assist him with, but he's made his bed and he's pretty much...