Hi guys need your help!

So my last period was 9th Nov but it was very light one lasted only 2 days (normally it is 3 days with light spotting 4th day) & was very light flow too.

Now yesterday evening I started having lower back pain which I ignored since I have had a long & tiring day. But at around midnight when I got up to pee there was little brown shade blood in my underpants and also in bowl.

I’m not sure what is this. Though I didn’t have any blood after that. I’m still having on/off dull pain in lower back. Also I have somewhat irregular cycles (varies from 26-36 CD sometimes). I don’t if this is implantation bleed or old blood from last period since it was a light one.

PS:- I have never had mid cycle bleed.