Almost certainly the lemonade. Plus, if you ever get an unexpectedly high reading, wash hands then try again - it could be spilt sugary liquid or juice from handling fruit.
Oats / oat bran etc - well any grain products really even the 'good' grains - are a major problem for me too. I was thinking 'oatcake' until the poster said she had had a glass of full sugar lemonade.