Doesn't sound concerning to me. She has great diaper output and has nearly doubled her weight by 3 months. Weight gain tends to slow down a bit as baby gets older- the average weekly weight gain for a breastfed baby in the 0-3 month range is 5-7 oz (reference: http://kellymom.com/bf/normal/weight-gain/). In the 4-6 month range it's 4-5 oz per week. But remember that these are averages: some weeks a baby may gain 10 oz, others 3. Growth does not happen at a constant rate in all dimensions. And highly active babies tend to decrease their rate of weight gain a little faster than sedentary ones. Finally, it sounds like you're weighing baby on a home scale, correct? Those scales are generally not accurate enough for measuring weight gain. When you're trying to get an accurate picture of wright gain, you want to always weigh baby in the nude on the same scale as last time, and that scale should be a precise, professional one.