It is best to get advice from your PS. Only he/she will be able to determine the possible cause. It looks like there have been some full thickness wounds on your back that are healing and your PS will be able to advise on dressings / wound management.
An issue like this needs to be addressed by your surgeon. Hopefully they have already told you what may have caused this and how you should be taking care of this. Any compromise of blood flow or circulation and/or trauma can cause this. Since there are a variety of ways of managing this, it is best to have your surgeon directly involved.