Why Would My 3 Year Old Son's Platelet Count Be

Why would my 3 year old son's platelet count be 550 with no other symptoms or any other blood hikes, 2 weeks ago it was 617??
Overview :
Four and a half weeks ago he began having high fevers of a 102° for four days they said he had strep throat tested positive at is primary doctor and negative at the hospital the same day. they gave him antibiotics( keflex), his fever went to 105°.... unrelated to the fever, temp went back to normal, then he had what they call a seizure, I'm not so sure he had a seizure, they did an MRI and found nothing, it was weird, and now no symptoms at all of being ill at all no other blood hikes just his platelet count what could it be? I'm really concerned....