The note said she was pregnant but "everything was gonna be OK, mom" and that she and boyfriend Casey Aldridge were going to raise the baby.
"I thought it was a joke," Spears said in an interview. "... I kept waiting for the punch line."
Spears said she realized it wasn't a joke because Aldridge, a pipe-layer from Liberty, Miss., wouldn't look at her.
"I was just truly in shock and then I started to cry. And then she started consoling me at that point," said Spears, who writes of Jamie Lynn and her older daughter, Britney, in her new memoir, "Through the Storm."
Lynne Spears said she blames herself for what happened.
"As a mother, don't we always blame ourselves? I mean, we're the first one that's gonna take the blame," she said. "We never get the credit, but we always question ourselves and say, `What did I do? We had this talk just a month ago.' I mean, I had no idea. I took a lot of the blame. I took all the blame."
Jamie Lynn, who starred in the Nickelodeon series "Zoey 101," announced her pregnancy last December. She and Aldridge welcomed daughter Maddie Briann in June. The couple are now engaged.
Lynne Spears said Britney is now back on track. The 26-year-old pop star recently picked up three awards at the MTV Video Music Awards in a recovery from her sluggish, performance at last year's event. She announced Monday that she'll release her new album, "Circus," on Dec. 2, her 27th birthday.
"She's strong, she's resilient, she's had some big bumps in the road but Britney's fine," she said. "Britney is headed to the top once again."
NBC is owned by the General Electric Co.
On the Net: