I Have A Red Itchy Welt With Blisters About The

I have a red itchy welt with blisters about the size of a nickel but seems to be spreading and getting larger. It is on my skin forward of my arm pit. It started three days ago and I thought it was a bug bite so used neosporine. It's getting bigger and now blisters have formed. Can you suggest something? I'm a bit scared and, yes, have been under lots of stress :-)