You say that you were very under weight at 7.5 stone, and now are 9 stone. You didn't mention your height, but I'm just under 9 stone myself 5ft4 and I'm skinny indeed...
The weight gain could be more to do with your body returning to it's normal weight, what you need to do now is to keep a close eye on your weight to see that you aren't increase above your normal weight for your height.
I would find out your height, look at a BMI chart, to determin where your weight for your height should lay..