Actually, exclusive breastfeeding doesn't guarantee that you won't get your period. It just decreases the likelihood. Your period and your fertility can return at any time. While many nursing moms will not experience a cycle for months and months after their babies are born, some will get their cycles back within weeks, and at the other extreme, some will not get their cycles back until their babies wean completely.
This could be your period trying to come back, or it could be an extreme outlier of your lochia (postpartum bleeding), or if you are on hormonal contraception, it could be a side-effect of that.
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!"