I have a weird looking mark on my stomach...help?!?

I was at the beach for the whole week last week. Monday was the first day I went in the ocean, but before I went in I asked the lifeguard if there were any jellyfish in the water because I am very scared of jellyfish. He said no because the water is too cold and it's too early in the season, and he guarenteed there would be absolutely no jellyfish. I went in the ocean and after about 15 minutes of being in, I dove through a wave and right when i came out from the water I looked down and my stomach was bleeding. In the center was a red cut that was bleeding, and around it was dark red and blue marks. Now it's 5 days later and there's still just a little blue mark on my stomach. Originally, the cut was about the size of my fist. This was definitely not a jellyfish sting. Any ideas of what could have happened? I did not feel anything bite me but I'm not sure.
 
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