At that age, it usually is more about growth spurts. My kids are younger, but yes, they absolutely get chubby right before the grow. They're chubby one day, then taller and lanky the next.
Unless there is a really problem with unhealthy food and/or severe lack of exercise, kids at that age really shouldn't be overweight. Their metabolism is much to high. That's why most of us reach a point in our lives when we suddenly have to start watching what we eat and exercising. It's not that we were so healthy before, it's that our metabolism has changed and we don't just automatically burn off the calories.
That's also part of the reason obesity is such a huge problem. Children simply should not be fat. It's not good for an adult, but it's very scary in children. Their bodies are supposed to be able to protect them from gaining too much weight, so if they are gaining too much weight, there is a serious problem with their lifestyle.
Eating healthy and having plenty of time to run around is important, but she's fine.