Ay, that's actually pretty common kahit naka-pills ka, mama! A few of us have experienced spotting or breakthrough bleeding even while on contraceptives, it can happen especially if may stress ka, irregular timing ng pills, or sometimes the body just adjusts differently after childbirth. Since you've been consistent for over a year and this is the first time, I'd really suggest checking with your OB to make sure everything's okay and walang iba pang reason, kasi peace of mind lang yan and they might need to adjust your pills or check if may other factors. Don't worry too much, but better safe than sorry talaga!