It was a legendary weapon I took from one of those Stone Tiles that require a feat of strength. I don't remember its name, but I remember it was a 2 handed axe that doubled my dmg output. I think the glitch started to occur right after I picked up a potion. I don't remember its name either, but I think its name was glitched (?)
Sorry I can't help you more, I have no idea what could be causing this. But it was right after picking an item, that I'm pretty sure.