Ann some babies start to plateau around this age as they are really active and burn off any extra energy with crawling etc.

Is she happy and strong and developing ok? If so, and she's eating well, I wouldn't worry too much. If however she is losing weight, and crossing percentiles, then I would go to a doctor.

At the end of the day follow your gut - mother's instinct is there for a reason and no one else knows your bubba as well as you do.