Dear Wendymlb: Without evaluation, including physical examination, it is impossible to speculate on what this might be. It is possible that this could be a hematoma (a bruise caused by bleeding under the skin). If it is a hematoma, these can take a long time (months) to resolve. However, if she has not been seen, she should be seen to determine what this is. You should make an appointment for her to be seen by her surgeon or oncologist upon their return or with a covering doctor.
There is always someone covering when a doctor is on vacation. The office will know who she can see, if she is concerned.