Metformin does little in the way of reducing blood glucose. Go back to your GP and ask for the slow release version, it might be kinder on your tummy/bowel. The Metformin is a mild appetite suppressor, helps with liver dumps and is generally regarded as a safe drug but some people just cannot tolerate it.
Start testing your bg before the first bite of food then two hours later, the difference in the two readings should be no more than 2mmol, this will tell you if the foods you have eaten 'spike' your bg too high. Start keeping a diary of your meals and the readings and patterns will soon become apparent. The foods to cut back/avoid are bread, potatoes, rice, pasta and anything made with grains or white flour. Breakfast cereals are usually very high in carbohydrates and most likely will raise your bg too highly though some people can tolerate a small portion. Testing at random times tells you nothing. Hope this helps.