Sting at Blue Lagoon

Just out of interest really, I was snoring at Blue Lagoon a couple of weeks ago. I was swimming near these rocks and the current pushed me into them slightly, I remember thinking ‘the rocks are a bit sharp” – nothing else really. When I later got back on the boat I noticed my leg was red and felt uncomfortable. Then it began swelling and stinging.

I didn’t see any jelly fish and I’m thinking it’s most likely the living matter on the rocks which stung me when I fell into them. I was just wondering if anyone has had a similar looking sting and out of interest, do you know what it is I was likely stung by?

Thanks!