What does it mean for a wasp sting swelling to *return* 2 days after the initial bite?

Swelling return: 2 possible causes of that swelling could be a small hematoma(blood collection from bleeding from the bite) or an early collection of pus which would usually have associated redness and tenderness. I recommend applying warm soaks and observe for spreading redness or more swelling which would warrant an examination by a physician. Apply antibiotic ointment and If itching occurs take an antihistamine