side effects after mastectomy

It's a big operation; chest infections and fluid collections are a major risk after any lenghty surgery. If you are otherwise well--which is hard to judge sometimes when you take into account the anaesthetic, the surgery, pain meds and the huge stress both mental and physical--they will keep an eye on you and maybe prescribe antibiotics. I've had a similar thing happen, antibiotics, some coughing exercises and gentle stretching sorted it in a few  days.