Mysterious huge very painful lump/bump/welt on my back?

i was sitting in a hard chair and felt some pain along my bra line on my back. I discovered a small redbump like a welt, or mosquito bite. it was painful and very red. I just ignored it figuring it would go away or maybe a pimple that would heal itself, but it's only gotten bigger and more painful. it's now in between the size of a maraschino cherry and a small grape. I was outside in a tank top that i normally don't wear tanks, usually tshirts or something. there were wasps around but I think I would have felt it, and I think we have seen a flee or two every now and then on our yorkies but they sleep in the living room and we've been flee dipping them. it's not coming to a head just feels like a very sore swollen large bug bite. any ideas what it could be?