Building muscle
 

I have experienced a 2.5 lb gain. It is surely due to exercising. When you exercise, how fast does your body build muscle?

This post in the General Exercise forum should answer your questions, Em. On 'average' it can take about 6 weeks to 'grow' muscle, but when you workout then rest you're involved in the process of creating muscle, and this can cause a scale increase right from the get go.

HTH
Nat