Q. Our little girl, three months short of 5, has suddenly changed. She has always been a joyful, even-tempered, well-behaved child, but now she's whiny and clingy, often unreasonable, and overreacts to almost everything.

She doesn't even want to leave home to go to her beloved school or to see her friends, although she likes to have them over. She has also been rude to me and her father and has hit him and some of her friends, but claims these are "love pats."

When I tell her she's looking for a major confrontation, she quickly turns into an angel, so we haven't had a showdown about her behavior.

I work at home, so she has had few sitters, and she was breast-fed until she was 2. Although she still covets a bottle, especially at the end of an exciting day, she acts quite mature for her age, and she's never been a discipline problem.

I am pregnant and I've been put to bed twice, but my mother-in-law takes great care of us.

A. Your pregnancy is bound to be causing your daughter a little tension. No matter how helpful your mother-in-law has been, you and your husband must be feeling anxious, and children reflect the anxiety of those around them. This probably isn't the source of her scenes, however.

The new baby shouldn't be bothering her either. She'll be a bit jealous right after the baby is born, and quite a bit jealous a few months later, when the baby's coos and goos will win every heart, but the baby is unreal to her now. Young children only believe what they can see and hear and touch. Your little girl is probably reacting to a more basic problem: She can't obey her age and her parents at the same time.

Children grow in sequential stages and though they don't all react at the same time, there is a definite pattern and an average timetable. Generally their behavior stabilizes around the time of their birthday, improves a little more each month, and then wham! It breaks apart about six months later. It's as if they have to enclose their emotions in a chrysalis to keep them in check, and then they have to shatter the shell, so they can grow some more.

And the greater the growth, the bigger the explosion -- which explains why 4 1/2 is almost as explosive as 2 1/2. Both are such expansive, extravagant ages.

Not all children respond to these stages with the same intensity, of course, and it will probably be milder for your little girl, since she has an easy temperament and parents who know how to handle her. You're in for a slightly tough stretch, however, but you can figure out what triggers some of her outbursts, so there will be fewer of them.

Most children are more susceptible at Arsenic Hour -- that miserable time between 5 and 7 p.m. when their blood sugar and their energy are equally low. A high protein snack around 4 p.m. will help, and so will an easier schedule, an earlier supper, and a lot of late afternoon hugs, whether she's been good or not.

Your daughter also needs you to negotiate more, and to give her more choices, but she still needs limits, even though she pushes against them. Different techniques make it easier for her to obey.

She'll respond better if you teach her new ways to do old jobs, but call them "tricks" instead, which sounds more enticing. You might also use hand puppets to give your orders and when you do lay down a rule, call it a tradition -- as in "It's the tradition in our family to ... " Four is the age of magic and you capitalize on it when you make the mundane sound mystical.

You can -- and should -- ignore some poor behavior now, since too many corrections can undermine a child's self-confidence and even condition her to throw a scene just to get your attention. Children often stop their foolishness quicker if it gets no reaction at all.

And when your little girl still acts outrageously, she deserves that most effective discipline of all -- the stern eye, and the low, no-nonsense order, "You will go to your room, and you will stay there until you can be polite."

Also, look to the future. As long as you give your child plenty of attention and affection when she's pleasant, she'll soon lift up her little bushel of gloom and let the sun shine through. Five is a wonderful age, especially in contrast to 4 1/2.

Questions may be sent to P.O. Box 15310, Washington, D.C. 20003.

Worth Noting

The Children's Theatre of Arlington will present two two-week theatrical workshops for children -- ages 5-6 and 7-9 -- between June 18-July 20 for $85, and a three-week workshop for 10- to 14-year-olds, from July 9-27 for $215. Call 548-1154.