Hypothyroidism is when your thyroid does not release enough hormones. This can cause you to have a slow metabolism - and to not lose weight properly.
Your thyroid is just at the base of your neck, in the front. It creates thyroxine (T-4) and triiodothyronine (T-3). These control how your body burns up fats and carbs. If this isn't working properly, you are stuck gaining weight and unable to burn it properly.
If your thyroid not sending out hormones at a proper rate, it can cause you to gain weight, become depressed, have sore muscles, have stiff joints, and much more. Some people think they are just "getting older" when really it is a medical problem that should be fixed.
Sometimes an improperly working thyroid is simply genetic, but it can also be caused by a lack of iodine, radiation, medication that has lithium in it, and certain autoimmune disorders.
There is a blood test that can determine if you do have hypothyroidism. If you feel that you might have hypothyroidism, a good first step is to calculate what should be your daily metabolism. A link below will bring you to an online calculator. Once you figure out how many calories a day should have you "maintain a weight", be very deliberate to eat at that or below for a solid week.
Make sure you count every single thing you eat, and track your portion sizes. Also make sure you accurately count your exercise. Walking a quarter mile is not usually countable as a measurable exercise for a day :)
If you still gain weight, it would definitely be time to talk with your doctor about having a blood test! It is something they can do quickly and easily, and it can clear up for you if there is a thyroid problem going on. If there is, there are very helpful medicines they can give you to handle the problem. If it ends up not being thyroids, you can narrow down the medical issues to start looking into.
I have many articles here on metabolism and how to raise it. Be sure to read through them and learn more about your own body's metabolism!