Re: How often is normal?
It is very normal for a baby who started out only nursing on one side to eventually transition to feeding off both sides. It is also very normal for a baby to drop some percentiles as they become mobile- all the calories that used to get packed on as fat go into motion, instead. My pediatrician claims that it's because breastfed babies "can't walk around with a bottle hanging out of their mouth all day long.".
Have you considered trying to offer the breast a bit more frequently? 3-4 nursing sessions per day is on the low end, in terms of frequency. And if you are at all concerned about weight gain- which I don't think you need to be- breastmilk has more calories per oz than solids.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"