Doctors have said their injuries are most likely to be caused by chemical burns, but it could also be algae or jellyfish.
Rebecca Curwood, who went in the sea, said: "We came out and we were stinging. Then we had a rash - we went home, put cold water on it and washed our legs."
Now people are being told not to go in the sea until the local authorities find out what caused the burning and rashes.
Scientists are examining samples of the sea water to find out what happened.