Can new sand flea bites show up days later?

Yes.: Sand fleas, as well as other biting insects, can trigger a delayed reaction at the bite site by a similar mechanism that causes the well known hypersensitivity reaction of poison ivy or poison oak rash, often as late as 2-3 days later. And similarly, an anti-inflammatory cream such as Hydrocortisone may blunt the reaction thus making it more tolerable and less itchy.