I think a reasonable guess would be jellyfish larvae, also known as "sea lice."
Sea lice are actually the microscopic larvae of jellyfish and other
ocean stingers which contain the same nematocysts (stinging cells) as
mommy and daddy.
(They) are probably the most commonly encountered stinging threat to divers and swimmers at the beach.
There's a wide range of symptoms:
Common symptoms of sea lice stings include an intensely itchy red rash
with small blisters and elevated areas of skin. These painful and
unsightly lesions may appear anywhere on the body but are often
concentrated in areas covered by swimwear as large numbers of larvae
often become trapped in these regions. Severe reactions to sea lice
exposure may include fever, chills, headaches, nausea and vomiting,
especially in children. Extreme allergic reactions may require
hospitalization in rare cases.
I've used these resources from "Buy Safe Sea" but googling "Singapore sea lice" will bring up many references to a stinging sensation from these tiny creatures.
Make sure to read this article about Seabather's eruption.