in Twilight she hadnt really grasped just how the venom felt until she had experienced it for herself.Hearing it from Edward is different from experiencing it first hand.In BD part 1 she remembered all the stories and was silent this time around at least on the outside she appeared to be,but inside she was screaming like she was on fire.
In Twilight, Bella reacted normally because other than the bite, she was healthy and alive. In Breaking Dawn, she was completely still, because she was already mostly dead. Or unconcious at the least. Edward's venom was basically her last hope at any kind of life.
If I remember correctly the book (I haven't read it in a while) Carlisle put her under anesthesia so she was asleep but still in pain. It was an experiment to see if anesthesia dulled the pain. She told Edward it was fine but in reality she was in pain and told Carlisle the truth.
in breaking dawn the movie right before Rosalie looses control Edward injects Bella with mophine , numbing the pain although it wont last very long , Edward quets " Rose , pass th mophine . " and " No , Rosalie let the mophine spread . "Also Bella knows its not good to scream and it would just couse Edward worse pain then hes already in so she remains with her mouth closed
In twilight she was normal coz the venom didn't go around the body before it went edward sucked the venom and purified it but in breaking dawn the venom went all over the body and she was still coz she was half dead and Edward bit several places so the venom was easily spread thats why she was still in breaking dawn and normal in twilight.
