I think you should be concerned and that you should tell your doctor because any rapid weight gain is lame.

I gained 10-15 lbs overnight (over 3 months) and have sustained that weight for about a year now. (Still on anti-psychotics) I would be weary of the weight gain because it could be messing with your thyroid and destroy your metabolism.

If you can, ask to switch to something that is weight neutral and drink plenty of liquids. (Without sugar or high frutose cornsyrup) Drinking plenty of liquids has actually helped me maintain weight. It also clears up skin if you're not stressed. ;3 Also working out sometimes helps.