Ay, hindi ka mag-iisa sa ganito! A few of my mum friends who are also on the pill after giving birth experienced the same thing, yung irregular bleeding especially in the first few months, kaya totally normal pa yan. Since negative ang PT mo and you're on contraceptive pills, it's likely just your body adjusting pa lang after delivery and the hormones from the pills, but I'd still suggest you mention it to your OB just to be safe, especially if it keeps happening or gets heavier, hindi lang peace of mind, pero baka kailangan lang adjust ng brand or dosage ng pills mo.