When changing a dogs food it is quite common to see symptoms such as diarrhea and bad gas. As I’m sure you know this kind of change needs to be carried out very gradually over several weeks. Dog food with a high quantity of cereals especially corn and soya are notorious for causing flatulence. You should look for a dog food whose main ingredient is meat such as chicken or lamb. If it doesn’t clear up in in a few weeks then it might be worth comparing the ingredients of the old and new dog food and seeing if there is a particular ingredient in the new one that might be causing the problem.