Yeah i wouldnt do leg day after back day personally.

Things i also find affect strength is the amount of carbs i've took in and the time of day i train but by far hydration is the biggest thing that makes me lift shit.

Make sure you're drinking like a fish, loads of water into you.

Sleep could be a factor too, or if work has been particularly hard recently? Maybe just burnt you out a bit.

Probably wouldnt hurt to go for a week of higher reps/lower weight either, get the muscles shocked a bit and see if you can re-bound back the next week. Just mix it up a bit.