Rash on chest that keeps changing??

Last Wednesday I had a spider bite on my leg and it was very hard and felt like a bruise. Kind of a raised bump with white-ish in the middle. A few days later, I got what looked like another bite on my clavicle. It was just a big red lump. Then I noticed a rash started to break out, mostly on the lower part of my chest, and it kept getting better than worse throughout the day and was a little itchy. The next day it was pretty much gone. Then that night it was a rash of rings, shown in this picture:


This morning when I woke up, the rash looked like pimple looking dots scattered around my chest:


I have had a rash on my chest twice in the past 4 months or so, specifically around my clavicle. They were not nearly as big, however, and was one targeted area of raised red bumps each time. I'm wondering if anyone can help! I haven't used or eaten anything out of the ordinary that I can think of, and I was tested for most common allergies two years ago and came up negative for all. I don't know if it has to do with the spider bite I had gotten either, since that was on my leg. I tried to go to the doctor, and all they said was that they didn't know what it was because it could be anything and to take Benadryl. Any input would be greatly appreciated!! Thanks!!