What stung or bit me on the beach in St. Lucia?

I was walking on the beach at a resort in St. Lucia, the Caribbean just as the sun was about to go down and while noticing all of the and what I umed to be sand fleas (but wasn't quite sure, their heads looked spiky and they were big, never seen anything quite like them before), I all of a sudden got stung QUITE PAINFULLY on the bottom of my foot (I actually cried like a baby). I hurt so bad that I had to immediately sit down as I couldn't walk on my foot after being stung or bitten, and there was a semi-large red welt where it happened. One of the guys who worked at the resort looked at it and told me it was probably a bee sting as at that time of day I guess they will fall onto the sand after the wind's been blowing all day because their wings are broken and I just didn't see it and stepped on it by accident. I don't think it was a bee or wasp as there was no stinger, I just know it hurt really bad and that it kind of felt like their was poison going into my foot. I've done some research and I now know that it could have been either a sand flea, dead jellyfish, or a tentacle of one since there weren't any on the beach visibly, or God knows what else. I also got violently ill that night at around 3 in the morning with vomiting, diarrhea and weakness. Have no idea if the two incidents are at all related. The swelling and pain did go away after about 15 mins, and after putting ice on the site, but now it's starting to itch like crazy after returning home from the Caribbean 10 days later. Was wondering if anyone knew what could have stung or bit? It definitely was a very stinging and painful sensation but don't really know if it was in fact one of the many sand fleas or whatever they were I saw on the beach. Thanks a lot.