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Levi Johnston's Mother Speaks Out

By Lorenzo Benet

01/08/2009 at 03:35 PM EST

Speaking for the first time since her Dec. 18 arrest on charges of selling OxyContin, Sherry Johnston – whose son, Levi fathered a son with Sarah Palin's daughter, Bristol, last month – tells PEOPLE she got hooked on the prescription opiate after a hysterectomy eight years ago, which led to seven additional surgeries.

"I was in pain, and I'm still in pain," she says.

Johnston, 42, pleaded not guilty at a Jan. 5 arraignment in Palmer, Alaska. "I have been trying for years to deal with the pain," she says. "I have a medical pump inside me. I'd rather not comment any more but it's not everything that they're saying."

Johnston said has been under a doctor's care and that she was prescribed OxyContin as a painkiller. Still, the hysterectomy and the subsequent complications forced her to give up her career as a hairstylist because she couldn't comfortably stay upright. "I am disabled," she says.

Grandson Lifts Spirits

Lately, Johnston's spirits have been lifted by the birth of Levi and Bristol's son, Tripp Easton Mitchell Johnston, who arrived Dec. 27. When she saw the baby the day he was born, "I started bawling," she says. "I think he looks just like Levi."

As for being a grandmother, she says, "I'm very excited. Absolutely."

Levi, 18, recently left his job in an electrical apprenticeship program in Alaska's North Slope oil fields after a newspaper columnist raised questions about whether he was eligible to participate. Johnston says her son and Bristol, also 18, plan to wed this summer.

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• Levi & Bristol's baby and wedding plans!