That happened to me a while ago when I was walking barefoot on a rocky beach. It kept opening whenever I'd take a step, so I had to awkwardly walk on the side of my foot sometimes.
YUP! I was walking barefoot on the sidewalk. I don't know what got me but it hurts lol. It's right where I step so I have been walking on the side of my foot, but now that hurts too lol