was it a box jelly?

hi, my name is dimitris and i had a bad experience with a jelly fish. It was many years from today but it was so painful that i will never forget! It happened at "thermaikos" area at greece. That day was was a bit windy and the waves had made the sea a bit blur. I was swimming with a mask and a short sleeve diving suit. As i was swimming i felt a terrible pain in my arm and it felt like a knife that had cut my flesh and stopped at my bone. I god scared, looked around without seeing nothing, my arm had no visible marks at that time but the pain was unbearable so i decided get out of the sea. At my way out, the same thing started happening again and again, my heart started to beat like crazy (i couldn't understand if that was from the poison or the pain). At last one of them got in front of my mask and saw what it was. It looked like a transparent dice at the size of a fingernail and had four long strings hanging from the bottom. The coast wasn't far but i knew that if those attacks kept on i would not get out alive for sure, so i started to swim underwater at the bottom of the sea, they seemed to be at the upper half so i managed to get out! i knew i had to go to the hospital but all i wanted was to calm down, all the uncovered skin had marks. Fortunately after thirty minutes i was a lot better somehow.
I know a lot about animals and had dozens of pets, i've been stung several times from a big number of venoms and i am no ignorant when it comes for those matters. I also know that its a bit difficult to meet this kind of animal in greece but if that wasn't a box jelly then what was it?