I am experiencing weight gain. What could be causing it?

Many causes. Weight gain can be caused by a multitude of reasons. Medications such as some antidepressants and hormone replacement can cause weight gain. Smoking cessation and sedentary lifestyle are also culprits. More serious reasons can include hypothyroidism, diabetes, renal failure or congestive heart failure. A simple visit to your doctor and lab tests should be able to diagnose these conditions.