Hello.
I looked up ringworm in the Chicken Health Handbook and there is no listing for it. I think that Gail Damerow is pretty thorough so if chickens got it, it would be there.
I also looked up breast blister which was the first thing that came to my mind when I saw your bird, and will relate what it says. It may be somewhat unusual in a pullet, but who knows what they do when we're not looking.
Symptoms: in growing or mature cocks, particularly of the heavy breeds, large blister on keel that eventually becomes a callus or scar.
Resembles: Infections synovitis, except an uninfected blister is filled with clear or bloody fluid, an infected blister contains creamy or cheesy material.
Cause - irritation and inflammation due to pressure of breast bone against roost or wire floor
Transmission - occurs in birds with weak legs or with poor feather development offering no breast protection.
Treatment: Open and drain blister. clean with iodin, and pack with Neosporin (not with the 'caines), antibiotic treatment in drinking waer or by injection may be necessary if blister becomes infected wity Mycoplasma synoviae (infectious synovitis) or Staphylococcus aureus (necrotic dermatitis).
While it may not be a ruptured breast blister, perhaps the treatment might work.
Good luck, Mary