Chemotherapy is intended to be a short-term treatment, and your body should return to normal afterwards. However, some types of chemotherapy can have long-term effects, particularly those that affect the
hormonal system. Some women who take chemotherapy drugs that suppress estrogen, for example, go into early menopause, which in turn can lead some women to put on weight. A cause of unexplained weight gain for some women is hypothyroidism, which slows your metabolism. Also, many medications, such as corticosteroids, can lead to weight gain. It also may well be that you are less physically active after going through cancer treatment, which is one of the most common causes of weight gain.
Make a list of all the medications you are currently taking and take it to your doctor. Talk to your doctor about your concerns, and ask for a thyroid test as well. It may be that at this stage of your life you need to decrease calories and increase physical activity in order to maintain your weight.