You most likely jumped over too quickly. Your system needs adjustment time, just like your muscles. You wouldn't go from couch-potato to full-blown heavy squats over night and not expect to hurt, right?
Protein often causes gastrointestinal discomfort, especially at first. First of all, I'd be careful of portion amounts. Try breaking up protein intake throughout the day. See if 20g makes you feel bad. If not, then try to do it 4-5 times a day, and work yourself up.
And don't eliminate your carbs altogether! Rather, make a transition to healthier carbs, like oatmeal, whole-grain bread, fruits and veggies. Otherwise you're likely to crash more often, or (even worse) quit the whole gig.
Take it slow, and remember you don't have to stuff yourself or sacrifice taste to eat healthy!
http://people.smu.edu/jowillia/recipes/recipes.htm