Question regarding the lagoon at Barra de Potosí

Greetings, Friends. On a recent outing to Barra de Potosí, I decided to swim in the lagoon. The only other time I had done that, the lagoon was open to the ocean. This time, the lagoon was closed to the ocean. There were other folks swimming in the lagoon so my assumption was that it was safe. After spending some time in the water, I felt as if I something was biting me. The bites weren't painful enough to warrant jumping out of the water immediately. Big mistake. Within a day, I was covered with a few dozen bite marks similar to mosquito bites. They itch tremendously. Has anyone ever experienced this or does anyone have any information about what these critters were?