Are you sure it is the Lemsip that has caused your spike?
If you are feeling enough under the weather to take it, there is a good chance, your liver is dumping glucose in your blood to fight off the lurgies - cold and flu typically raise BG without the assistance of drugs.
As usual, when feeling under the weather, the treatment is test, correct, test, plenty of liquid, test, correct, test, test again ... did I mention testing?