Sounds like you are eating & exercising exceptionally well whilst pg. You might just want to check that your museli doesn't have a high amt of sugar.

Your body will gain the amount of weight it needs to during your pg. You need to remember that you carry extra body tissue, placenta, fluid and blood as well as your developing baby. On average a newborn baby weighs approx 3.5kgs. 8 kgs seems well within what the 'normal' weight gain is. The higher than usual weight gains are probably your baby growing & developing.