Likely implantation???

I'm 13dpo today and had light spotting when I wiped. My cycle WAS/IS a little longer this month because I'm on CD37 when my average is typically 30 days. Still, I CONSISTENTLY start on day 17 after I peak LH no matter my cycle length. Even after a shorter 27-day cycle, the next started 17 days after my LH peaked which made it come earlier. Is it more likely that I'm experiencing implantation bleeding or that my luteal phase is off? I've read the follicular phase fluctuates which gives you a longer or shorter cycle but that the luteal phase is pretty steady. I was only able to bd once at the very end of my window so I'm trying not to be too excited or too negative.