You were obviously very, very VERY dissapointed judging by the dozen or so posts! :lol:
Muesli can be quite high in carbs, so it was almost certainly that. Some may say "liver dump" because of the exercise, and who knows, but it normally comes down to food. Try testing again after the same food and routine another day. If you still get the same'ish reading, have a low-carb breakfast, do the same exercise (if there's any Jasmin left by now!) and test again. You will then know for definite what caused it.