Stung while in river

bumps on skinHi, Today I was swimming in the river Ouse, Nth Bedfordshire, UK as I do every summer but this time something stung me on the water. Attached is the pic. Ankle and back of knee. I felt both stings very clearly and painful enough for me to say “ow” out loud and want to get out of the water. The white bumps came up immediately. 4 hours later they faded to red dots but the burning is still very intense. Any ideas on what it could be? Bit concerned, very curious. Glad it was me and not my kids.  Many thanks.  Sarah

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