LAID-up “Alias” star Jennifer Garner says she was misdiagnosed last month – and what was at first said to be a virus turned out to be pinched nerves.

She was injured, she says, while performing a stunt for her spy-TV show.

“I wasn’t feeling well over Christmas . . . and I never even get a cold . . . so I was a little worried that I had some kind of infection,” she says on tonight’s edition of the TV magazine show “The Insider.”

“It turned out that I had damaged a couple of nerves very, very slightly in my back doing some kind of stunt.

“You know, it’s hard to believe I haven’t done more of that to be honest . . . considering what I’ve done over the past year,” she said during an interview Saturday night at the opening of her new movie, “Elektra.”

For more than a week, studio officials had been saying that Garner was in bed with a virus.

Garner – under doctor’s orders to take it easy – canceled a hosting gig for the Jan. 15 episode of “Saturday Night Live.”

“Alias,” meanwhile, returned for a fourth season last week in a two-hour premiere that averaged 16 million viewers, the show’s best ratings in three years.