Hi there and thanks for your question. This will be difficult to answer without being able to see the lump, but I can give you a few of my ideas.
A good number of growths on dogs are benign and nothing to worry about. However, you have mentioned a few things that have me concerned. I don't like the fact that it is growing and that it has "parts that are unhealed." There is a possibility that this is a cancerous growth. Surprisingly, cancerous growths aren't always painful. Most benign growths will be smooth and small and regular in appearance. When we see ulcerations and irregularities then this is generally not a good sign.
Ideally it would be best to have your vet do a fine needle aspirate of the growth or even better a biopsy of it. The vet should be able to tell you if this is cancerous and if it needs to be removed.
Is your girl spayed? If not, or if she was spayed after she had gone through a heat cycle then she is at much higher risk for getting mammary cancer.
So sorry for the bad news.
I hope she does ok!
I hope this information helps! If you have more questions regarding this problem, just hit reply. If I am online I will respond right away. Otherwise I will respond as soon as I come online again. Or, if my answer is sufficient, then please hit accept. Thanks!