help- 3 month old and spots on leg

Nothing • 20 ✨ Christopher born 7.27.2016 💙 happy 💗🐰🐱
I just noticed these little red dots on his leg. Only on one of them. He's 3 months old. Does anyone know what this is? He had an allergic reaction to some antibiotics the other day but his rash didn't look like this.