Bleeding but AF not due for another 10 days! Why?

So I have been ttc for around 1 year. Since stopping BC My cycle went from 33 days long to 35 days long and now it's been consistently 39 days long for the last 4 months. I've been very regular with it and I always get it at the same time. However this month I bled on CD 29, which is 10 days earlier than usual and tbh quite strange. It first started in the morning as a small gush of pink blood, then a tiny bit for the rest of the day, only leaving little specks of pink . No need for a pad, just a panty liner. I had mild pelvic pain the night before for only a moment. And feel very very mild cramps today.

Ive only bled early once before but that was when I stopped my BC and my cycle were still all over the place.

Anyone have any experience with this? Or have any idea why this might have happened?