What stung/pinched/stabbed me while at the beach?

I was at the beach (Gulf Coast) and was teaching someone how to surf. As I was walking her in I fell into a large pit. I went from thigh deep water to neck deep. As I tried to get out of there I felt something hard brush against my ankle. I thought it was a rock so I didn't move but then I felt a sting. At that point I moved my feet away from there and could still feel the "rock" moving towards my foot. Right after it happened my ankle was very sore kind of like when it's sprained. When I stepped out of the water I saw a tiny little spot that was bleeding right over the bone on the inner ankle. It was really sore all throughout the day and night even after I soaked it in hot water (7 hrs later). The pain even radiated towards my knee occasionally and I was actually limping around yesterday because of it. Any ideas as to what could have caused this?