My child's leg suddenly turned red. What could it be?

My child has a red bump on his leg. Is this a symptom of hives? Since it doesn't appear anywhere else, like his stomach or arms, I don't think it's caused by food... Does anyone know if this is the case? I'm wondering if the syrup my baby is taking will help with the rash. If anyone has had this symptom in their child, please share any information.

It's redder than in the picture.