Allergies don't always come immediately. Not saying that this is what it is but you can take Tylenol 10 times and on time 11 have reaction to it. Doesnt have to be an immediate thing. Allergies come overtime, it's a possibility. What type of diapers?

@beebaby, his thighs but he's gotten both shots before I believe both were his 2nd series of them

Where was the shot given?