It may even just be something outside your workout plan.

For me it was the treadmill. I usually ramp up my training in early January and by Feb or March I'd be be dealing with my knee (Patellafemoral). Because I'm in Canada, a lot of the early training in winter was/is on the treadmill. This year I connected the dots - it wasn't the volume so much as how I did that volume - specifically a cheap treadmill that has a bit too much cushion. What I suspect is that I'm overextending relative to pavement.

This year, I've done it almost all outside, even in deep freeze or blizzard and I'm able to handle more volume than I ever have. Now that I've gotten my base, I can even go back and do once a week on the treadmill without risking it all. Though now when I do go to the treadmill I both check that it's a good one and I make sure to change my incline or pace at least 1x per minute (so I'm not doing repetitive kms over and over)