Bleeding after 15 days of period. Pid?

I had normal delivery in 2018. I'm not feeding baby now. I had my periods after 1 month of delivery. From the last 4 months, I had no periods. But lite bleeding used to be seen for 5 to 8 days, for every interval of 15 days. I had regestrone and I got my periods 19days back. The period was very very heavy and I was very weak those 3 days. I have hypothyroidism, for which Iam using 100mcg thyronorm for the past 4 months. As per doctors advice, I went for pelvic scan and serum prolactin. Results are normal. Thyroid TSH is 1.7. My worry is, after 15 days of period, I see bleeding, like the one we see on 4th/5th day of periods. I have the bleeding from 4 days, with slight abdomen pain and back pain. What might be the reason? Do I have PID ?