If you are exercising more the nit sounds like you are increasing muscle mass. At 98 pounds I would guess that there is not much body fat to lose as a result of your more active lifestyle - this can often mask the weight gain and what is gained in muscle is lost in fat. This is one reason why mass alone is not a good indicator of progress for people taking up exercise (muscle is much more dense than fat and you can actually gain weight whilst losing inches). Watch your diet to allow for the increase in protein consumption and consult your doc if you are concerned.