Sorry, but you are ill-informed! Chocolate/caffeine is a MAJOR trigger for migraines.
However, if you are NOT a migraine sufferer, and you are addicted to caffeine, then you could be experiencing headaches as part of withdrawal symptoms when you don't consume enough caffeine. Headaches are a very common withdrawal symptom involving caffeine, alcohol and other substances.
From the sounds of it, you suffer from withdrawal headaches not MIGRAINES.
By the way, what in the world are you doing consuming soda and caffeine if you have IBS? This is equivalent to shooting yourself in the gut and wondering why you aren't feeling well!