Period 10 days early - why?

Hi,

I’m 31 with diagnosed stage 2 endometriosis (this was a few years ago now and unsure of the damage now).
I’ve always had very heavy and painful periods, but regular.
My partner and I are TTC, and are going through the initial blood work for IVF as i’ve been told my chances of conceiving naturally are slim due to many factors including tubes are stuck and cannot move.
Whilst waiting for IVF we’re continuing to try naturally, not massively just tracking LH peak and that’s all. I had peak on cycle day 14. Cycle day 19 I started slightly spotting with very mild cramps, I did think (from research) this could be implantation bleeding. Cycle day 20 it because much heavier, with clots. For me, it’s still lighter than normal period but too heavy for implantation for sure.

Anyone know any reason why I could have gotten my period 10 days early? I’ve never bled between cycles like this before.