Does LP ever increase?

Just wondering if anyone might have some ideas. I thought that the the number of days after ovulation always stayed the same, but the number of days before ovulation can vary from cycle to cycle. My LP has always been 13-14 days. Currently, I'm on 16DPO, with negative HPTs for the last couple of days (including today), absolutely no pregnancy symptoms, and also no symptoms as though AF is on the way like I usually have. I had a strong +ve OPK 17 days ago together with EWCM, so I'm assuming I O'd maybe the next day, which puts me at 16DPO. I've never had a cycle this long - EVER.

Does anyone have any ideas what can cause a longer LP than normal if you're not pregnant???

Thank you :)