One condition known to affect weight is thyroid disease. A thyroid deficiency can cause a decrease in metabolism and may lead to weight gain. Visit these resources to learn more about the symptoms and treatment:
Thyroid Disease 101
Quiz: Could You Have a Thyroid Problem?
There are any number of drugs that may have weight gain as a side effect for some people. Some common ones include hormonal medications for birth control or menopause, oral steroids, some anti-depressants, diabetes medications and anti-psychotic medications. Wrong Diagnosis offers a comprehensive list of other medications that may affect weight.
You should get a diagnosis from a professional in order to determine whether your weight problems are medically-related.