Jelly fish sting in the beach

We actually didn't see the jelly fish but it stung our son while swimming in the beach. We saw a thread like thing (it must have been the tentacles) in his body while he was crying out of pain; we removed it quickly with our bare hands for his relief and then we too had the burning and itchy sensation - the pain lasted for around 20 mins. It was fine after we took bath in our room. This happened in South Padre Beach on 9/6/2014.