Hi Wits,
Excellent photos. It looks like you might have some retained fragments, for example, spicules from a sponge. You also may still be recovering from the effects of fire coral nematocyst discharge. In an acute sponge envenomation, it's effective to use duct tape to remove the spicules, but that would be questionable at this point.
If the rash continues to spread or changes character, i.e. pain, swelling, warmth, and/or fever, you should see a physician immediately to rule out cellulitis.
In general, topical corticosteroids and antihistamines can be effective in relieving symptoms like this. Please keep us posted!
Best regards,
DDM