Could be a growth spurt, could be teething, could be an adjustment in supply. Many moms who start out nursing on just one breast per feeding ultimately transition to using both breasts. I had a tremendous oversupply in the beginning with DD2, and needed to use one breast per feeding for several feedings in a row. Eventually my supply adjusted to meet my baby's demand, and since around 4-5 months, she has taken both breasts at a feeding.
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!"