my daughter reacts that exact way to ant bites/mozzie bites. Big round red hard lump with tiny yellow blisters on it. First time I thought spider bite and off to dr and she was put on AB's. The next time we just waited to see what happened, it went down after a number of days. Two weeks ago she came home with 4 of them on diff parts of her body. They have all gone down and gone now, but they're terribly itchy, red, hot and sore. But def worth getting it checked out.... Just in case.
With Jess, it's just how she reacts to them. My nephew is the same except he also gets hives all over as well as the local red lumpy thing.