Dog did fine one month on food and not the next?

I have an 8 year old German Shepherd/Kelpie, and last month I had him eating California Natural herring and sweet potato, and he did great with that. Then this month after buying another bag he has the most horrendous farts after he eats, literally every 10 minutes he is farting and its awful. I don't know what is different about this bag, but does anyone know if they changed their formula? Its not expired either. I don't have the last bag so I can't check the ingredients from the old one.