Hi Laura, welcome and thanks for your question. Proctor and Gamble does not list weight gain as one of the common or adverse side effects for Actonel.
Since we all react differently to this medication it may be possible for this to happen, but not very likely. Check with your Dr. to see if this is possible and if he or she has any other explanation for increased weight gain.
The Dr. may ask you to document your current daily diet so you might want to be prepared with a list of things you eat each day and on a regular basis. Also, it may help to do some diagnostic testing to see if you blood or urine tests show anything that may cause weight gain. Testing your glucose levels may be of some help as well, but your Dr. would have to decide to do this, or consider it something that should be checked.
Good luck finding out what is causing this, and also tell the Dr. the types and frequency of the exercises you may be doing as well.
If you have another question just post again, and I hope you find out the cause for this.