Jellyfish sting

I didn't actually see the jellyfish. I experienced sudden extreme pain on my leg from ankle to groin. I was in waist high water close to shore. I had huge welts and also patterns of smaller red dots over the entire leg. Still not completely faded over 3 weeks later. A hot shower causes them to redden. I was given an anti-venom injection at the time. Hotel called a doctor immediately so it's obviously a common occurrence. We saw some small white ones on the beach, but a local told me there are also larger blue ones that usually cause the bigger stings.