Definitely a bite of some sort! My mare had one earlier in the summer, just forward of her udder. It was quite worrisome initially as it was directly over her navel where hernias are often seen. It will take a few weeks for the swelling to disappear.
My horse had one in almost the exact same spot a few weeks ago. I emailed a picture of it to my vet and he suggested cold hosing it or let him know if it was making her uncomfortable - lets just say that I'm the only one that seemed to be made uncomfortable by it! It disappeared completely in a day or two. I suspect it was a bite.