'Next minute, she was a mother'; Winter Wife, by Jessica Auerbach. New York: Ticknor & Fields. 221 pp. $13.95.

''It wasn't marriage that changed people and changed relationships,'' reflects 26-year-old Amy Gold, a new mother, ''it was children.'' The shock of motherhood is doubly difficult for Amy, because she has no familiar surroundings or friends and relatives nearby to help sustain her.

When she returned to Connecticut from Maryland after her mother's funeral, she had blurted out a grief-stricken question: ''Haven't we been in this town long enough?'' And Paul, her husband, agreed, saying, ''If we've learned anything from your mother's death, it's that life is short.'' So they left New Haven and moved to Minneapolis, where Paul accepted a job as an administrator at the University of Minnesota, and where Amy, five months pregnant, found herself with nothing to do but await the baby. ''In my other life,'' she later says, ''I'm a flutist.''

They had been equals back in New Haven. ''Having a baby changes the balance of power,'' she tells Paul. He replies, ''You don't have to put it that way.''

The decision to have a baby was made as impulsively as the decision to leave the East; after the baby is born, Amy keeps trying to remember their reasons. They had decided to have a baby for fun, she recalls. It isn't. Winter has descended, and she is trapped in an igloo of an apartment, ice climbing the windows, her life dominated by the cries of a small demanding stranger who at the moment of birth irrevocably altered her life: ''What had been surprising was that it happened so suddenly. One minute she was Amy, the next minute a mother.''

Unable to cope with the baby, demoralized, she can't handle the simplest household chores, either. Paul's exasperation with her incompetence is growing daily. She turns on the television set to find Phil Donahue asking, ''Is the caller there?'' and an audience laughing at a desperate mother on the other end of the line whose toddler is ''always dumpin' out the Cheerios box on the floor.''

The person who comes to Amy's rescue is Eddie, the mailman who has been bringing her baby presents sent by relatives. The father of five, he proceeds to teach her motherhood, showing her how to bathe the baby, giving her confidence - and also his spaghetti sauce recipe. Amy's gratitude blooms into love, and eventually she begins concocting a fantasy in which they run away together to Virginia, to warmth and a springtime both actual and symbolic.

The few technical problems in this first novel are those that an editor should have corrected. A more complicated difficulty is the sudden switch from Amy's to Eddie's viewpoint after the reader has settled into expecting Amy's point of view.

Compassionate, often funny, this is an excellent novel, one that takes a distinctly individual look at people caught in a timeless plight.