jejenes

do not put bare feet or bury bare feet in the beach sands. There is a critter, commonly called 'sand fleas' that just love gringoes! The rash you describe could be the sand fleas. Or any number of other maladies-

I'm afraid you've got your vocabulary completely confused. You've confused "sand fleas" with "sandflies", because the only thing that could be called a "sand flea" here are the small crustaceans we call a chiquilique, but they don't bite.

What is known as a "sand flea" does not bite either nor does it survive long out of water.

What DOES bite that lives in the sand are what we call jejenes and what you call in English noseeums or midges, and they tear me up worse than mosquitos, though some people they don't seem to bother at all. They usually are out early in the morning at dawn and late in the afternoon, around 4PM or so. AUTAN is what we use locally to keep them off of us and it can be bought at almost any tienda or farmacia.

Autan has been hard to find, at least at La Comer and Bodega Aurrera. I used H24 in a spray can (43 pesos at La Comer) and it works fine.
The jejenes take cover in high winds, one of the pluses at Playa Larga....which is jejenes-Heaven on those unusual windless days there.