Identify this rash?

Hi think i should explain my situation here.
I have a dog (a terrier mix)thats about older then 10 years. When i got her at the shelter about 4 years ago i found out she has lyme disease, and cataracts. Lately over the winter i noticed she has been weakening so i brought her to the vet because she has gotten skinnier and had problems laying and getting up from naps. I figured it was just her lyme disease taking effect even worse. Aside from her age and disease i noticed a small rash under her front leg like near the "arm pit" of the dog. I figured it was just some rash and so the good doctor gave me gentimyasin or something, some kind of spray. I have been using the spray on her for almost 3 days now and this rash does not look any better. So i here i am thinking this does not look like a normal rash anymore it looks really bad and brownish! Now i am thinking its a tumor?? Iam really worried for my best friend here, i know i should call the vet back and i am going to but i am curious as to what other people think this could be, i enclosed some pictures of this rash, you can see theres a red lump near the top of the rash. I hope my puppy isnt getting older and starting to "retire" on me..