I've dived in Cozumel quite a bit and had similar marks, and the individual marks look like hydroid stings but the lines suggest jellyfish. I have had hydroid stings but those I know immediately when I touch them near the bottom of the ocean. The jellyfish stings I get are usually on the safety stop or more usual, at the surface wating to get on the boat and are usually a more solid line with less space between the bumps.
I get hydroid stings on my hands when I use my camera and above my ankles on the front.
I get jellyfish stings normally in a line that wraps around my calf. I don't seem to be bothered by the thimble jellyfish. Occasionlly I will get a small sting on the lip but those generally don't raise up and happens on the end of a dive nearer to the surface. Never on the neck for some reason. My wife is the one who gets jellyfish stings on the neck.
I dive in a 3mm jacket and surf shorts so legs and hands are the targets. For me, hydroid stings itch the most and can last one to two weeks until they go away. The jellyfish stings for me sting less overall but last longer and don't raise up as much. I used to dive in a short sleeved nylon t-shirt but after being sting in the armpit more than a few times, the long sleeve and tighter jacket is the solution.
OTC cortisone and topical antihistimine work fine for me for both jellyfish and hydroid stings. I have noticed on some trips, small pieces of hydroids in the water column but that is usually due to a storm or strange currents. I had someone we knew that had allergic reaction to sand flea bites in Cozumel and could not sleep until they took OTC oral antihistimine. I don't think these are sand flea bites.
Hard scrubbing hydroid and jellyfish stings with water and soap makes it worse for me.
YMMV (your daughter) for reactions.
I have found spring time over the years has more jellyfish but I suppose it depends on water temp and other factors as compared to winter months.
I would think that the sting would have been noticed when it happened. So if remembered, stings near the surface are probably going to be Jellyfish and those on the bottom are probably be hydroid. Fire coral is on the bottom also but having been stung once or twice in twenty years, a fire coral sting would be immediately noticed especally in three different areas.