Formula change to Alimentum

Hey all. Question! My newborn is 3 weeks old.. She just got switched to similac alimentum, My issue is that she is only eating half of what she usually eats, screams through feelings and yesterday she screamed from 3 PM until 10:30 PM. The only change was the formula. So we started using the old formula again again, which is similac pro sensitive. Today I am mixing the 2 formulas, and she seems to be doing better. I am hoping this gets her body used to the alimentum somehow. Has anyone ever done this to slowly transition into a different formula? I also can't tell if she is colicy or just miserable. Any help is appreciated!