Why shouldn't I switch formulas?

I went to the doctor today to just get a flu shot for my lil girl and I have recently switched from BF to formula and so I have been using the free samples I got in the mail before she was born. Sooooooo they vary from Enfamil to Similac and I was also wondering about the quality of Publix store brand and I heard Target store brand was equal to Enfamil or Similac and I just wanted to verify. The nurse giving the shot said I should just pick one and stick with it and not to switch them. I felt stupid asking why for some reason and now I regret not asking....does anyone know why I shouldn't switch? She has been fine with the tastes of all of them. I use Similac, Publix brand and thawed breast milk I had stored every day. I still have Enfamil in the cabinet and I am not going to waste any, cause its too expensive. I usually use the Publix brand for her cereal, but I also give her the remaining in a bottle after all cereal had been mixed. I thought maybe because of allergies, but I have introduced each one for a few days first and never at the same time as something else new (since she is eating solids now).

i heard its because it gives them stomach aches......its like switching a dogs food.....they should just be used to one

Because each formula varies slightly in its ingrediants it can sometimes cause stomaches, excessive gas, constipation or diahria.