What is this?

I woke up the other day with what looked like a bug bite on my arm. It doesn't itch but is very swollen. I drew lines to see if it was getting bigger, this pic is from yesterday and it's definitely bigger. It's a bit bigger today but not as red. We get a lot of spiders at my house so I was thinking maybe it was a spider bite? This is now the third day with it. Any idea as to what it could be?