Hi ladyluck67,
Unfortunately there are some medications which can cause weight gain, prednisone being one of them. Long term effects from these medications are dependent on dosage and duration of therapy.
You have mentioned that you watch what you eat most of the time, keeping a food journal can help you. Once you note what you are eating all the time it will be easier to make adjustments to your diet to keep you on the weight loss track Other than fat you need to monitor your salt intake .The type of exercise you do is also important, light resistance training and low impact such as yoga or tai chi can be beneficial to your efforts and help alleviate the pain and other symptoms of arthritis and fibromyalgia.
You may enlist your physician to help in your efforts as well as discussing your medication regimen and possibly making some adjustments.
Hope this helps.
Sophia De Monte RPh