Help, Does anyone recognize what this could be?

Taken last night about 8pm
Taken this morning at 8 am 12 hours later

Mackenzie woke up from a nap yesterday with this on her arm. Now the sore in the middle just looked like a small blister in the middle of the red. When I went to get Brook from school I had two friends look at it and they both said it looked like a burn, and then I asked the school nurse to look at it and she said it looked like a burn and said call the DR. which I was going to do. So when Rowena got home from school at 4 pm I called the DR. and they were so kind to let us come at practically closing time. When the DR. she said it looked like a burn and it was definately infected so Mackenzie is on antibiotic and neosporin gets rubbed on it. But the DR. says it is strange for a burn and she is not sure what it is. When the DR. saw it, it still just had a blister looking thing in the middle and the by 8pm the sore part had grown, at 12 am I checked it and the sore part had grown more, but not the red area. This morning at 7 am it has grown just a little more. I don't know it just doesn't seem like a burn. I have not seen a burn that eats away to top layer of skin. Now she had a little bit of a fever yesterday, but she ate dinner last night fine and breakfast this morning ok. She is not acting sick, she is playing and talking and acting happy. I don't know what to think of this. I am stumped. The DR. says it looks weird and she is not sure what it is. Has anyone seen anything like this? Or does anyone have any ideas of what this might be?