Check THIS out! We decided to treat our girls for mites today because we periodically struggle with them. When I picked up my lavender orpington I realized she had bumblefoot. When I turned her over, I found this nastiness! It was actually very simple to surgically remove and my husband stitched her back up. It was most definitely infection, it smelled horrible. She is currently in the garage and we are hoping for the best, but who knows. We also went ahead and cut out the bumblefoot while we were at it. The yellow thing in the picture is what we removed. It was probably about the size of two golf balls.
Y'ALL. I've dealt with some seriously nasty stuff in the past few years of having chickens but this definitely takes the cake. This is the same chicken that we had to treat for having a very infected dog bite. This bird either has 9 lives or is just horribly unlucky.
I'm happy to report that she is still alive today! I put some clindamycin in her water today and we will do wound care when we get home from work. It is truly astonishing what these birds can live through.
The sore on the breast is called a breast blister, it's like bumblefoot on the breast. It's entirely likely that the bumblefoot was a contributor. If feet or legs are sore they will rest with the breast on the roost causing rubbing, irritation, and ultimately the sore. Sometimes heavy birds will do it even if they have no foot/leg problems, just because they are heavy. Treatment, as you did, is similar to that for bumblefoot. For birds that do this habitually, wrapping the roost with something to cushion it can help, just make sure it's not something they can peck and eat. Hopefully once the foot heals she can roost normally and it will no longer be a problem.
Here is a post with some pictures of another one, post #82 here: https://www.backyardchickens.com/threads/reference-pictures-wanted.1295231/page-9#post-21093960