Hello,
From the symptoms it sounds like hives. Hives (medically known as urticaria) are red, itchy, raised areas of skin that appear in varying shapes and sizes. They have a tendency to change size rapidly and to move around, disappearing in one place and reappearing in other places, often in a matter of hours.
The next time it occurs,pls apply calamine lotion and consult an allergist to confirm whether your baby is allergic to anything. I hope that helps. Take care and pls do keep me posted on how you are doing or if you have any additional doubts.Kind regards.
May be an urticarial reaction,nothing to worry.