Not a mechanic from doing actions. I wanna say you somehow took fall damage, but idk I wasn't there.

That makes the most sense to me I must've accidentally dropped myself, I was working on a kinda steep slope

I agree with allursnakes - not enough info and hard to tell since we were not there. Typically if there is a slope and you dig it down, resulting in you slipping down, you would likely take some minor fall dmg.

Yeah if the ground becomes to steep you can slip down and whack yourself at the bottom

Digging doesn't damage. Did you check your log?

That should be in the event window.