It can happen anytime regardless of how long you've been on the pill. The longer you stay on it sooner or later it would happen at some point. It's the #1 most common side effect of being on the pill.
Thank you for the response. This is the first time that this has happened in the three years that I've been on it so I wasn't sure.
Keep taking it as usual. Breakthrough bleeding and/or spotting is a common side effect of being on the pill and definitely not a reason to stop taking it. In fact if you stop taking it you'll mess up your cycle schedule so I would not stop taking them.