Not drinking as much formula?!?!

My 6 week old daughter Grace has been a dream for us.. she is already sleeping through the night.. gaining weight.. drinking 6-7 4oz bottles of formula.. that is until two days ago.. since then she has been more fussy at feeding time.. she isn't drinking as much and has taken only 3 oz of most bottles (usually puts down 4oz no problem) and that is a challenge.. she appears to be having some gas pains (which is not normal for her) but I can't really figure out what has changed.. the amount, temperature, frequency, brand of formula have all stayed consistent.. anyone have any ideas? I am just worried that I am missing something.. thank you all..