This would be a great question to discuss with your primary care doctor
. Unexplained weight gain is really common, and they will be able to perform a general check up to see if there is any medial reason for your weight gain. In particular, one condition called hypothyroidism (under active thyroid gland) is a very common cause of weight gain. Other symptoms might include decreased energy and changes in your hair or skin. Hypothyroidism is easy to diagnose with simple blood tests.
If your doctor, on the other hand, doesn't find any evidence of a medical cause for your weight gain, then it will be important to reassess closely your exercise regimen and your daily diet. One excellent way to do this would be to have your doctor make a referral for you to see a nutritionist
. They can take a close look at what you are eating and determine whether or not you might be exceeding your caloric needs. This is an important step because people will often have the impression that they have not had any changes in diet or exercise when, in fact, they may have had subtle changes over a period of time that could translate into weight gain.