Just thought I would pop in for support because, yes, I had that problem!
For me, I guess it was stress. My cycles used to be about 35 days every cycle but I started to have 40+ day cycles, then some anovulatory cycles (50, 60, 80 days sometimes). Anytime I SWORE I was pregnant or thought about it too much, my cycles would be longer and longer. I could probably count about 6 or 7 times where my period started the same day, if not hours after taking a pregnancy test and seeing that it was negative. Of course, I have fertility problems so others probably won't have so much of an issue like I do.
Red Raspberry Leaf tea is a great regulator for me, although I would research it before trying it out. If I was having a particularly long, anovulatory cycle, I had my period within three days of starting the tea. Now I try to keep it a regular thing, although I do fall off the bandwagon a lot. It might just keep yours regular, instead of them shortening. Some women don't take it in their 2WW though, because it may cause uterine contractions for those with a "weak" or "irritable" uterus. https://natural-fertility-info.com/red-raspberry-leaf.html