What Causes Stomach Ache While On Metformin?

Hi, I have a question. My doctor started me on Metformin 3 weeks ago and I began with taking one pill with dinner. Then a week ago I began to take 2 pills a day (one with breakfast and one with dinner) and for the past two days I've been experiencing stomach pain. Should I be worried about lactic acidosis or is it just a side effect that will go away? Perhaps I'm not eating a big enough breakfast (I only get this stomach pain after breakfast)? I'm also on Synthroid.