What did you have?
How long have you been abstinent for?
I can't imagine eating one piece of protein would do all that, but I guess it could depend how long ago you ate something.
The only issue I have had is that I got to week 3 and I was still pretty constipated even with eating all that food, so LLC told me to add in fruit early, which did the trick.
Stomach-ache is probably the constipation, but I can't imagine how one item can make you constipated that quickly. It could be that eating something has triggered your bowels into extra movement as it actually has something to do now.
Foodpacks played hell with me so I think my body needed a bit of time to get used to eating and get going again.