R.M.
How long has she been eating less? You can always take babies into your pediatrician's office to get a weight check. Mine didn't require an appt, it was just a quick walk in for exactly this kind of thing. Maybe you could take her in tomorrow to compare with her last weight, and then take her in a few more days or a week or so. And keep track of the wet and dirty diapers, on a piece of paper would be good.
That is all just to be watchful. If she alert and happy this could just have to do with a growth spurt. My son has always had crazy growth spurt cycles, from the beginning. He would eat a TON, then he would stop eating so much and I would start to worry and he would sleep a lot, and as soon as he would slow down on the eating and sleep extra, I would notice his clothes not fitting or that he'd gone up a diaper size. Almost overnight. After a few days of not eating things would go back to normal and he'd eat a regular amount until the next growth spurt started. I felt like I was always on a nursing rollercoaster because I wouldn't be keeping up with him, then right as my milk supply would catch up to his demand, he'd slow down and I'd be miserable and huge. My son is 2 1/2 now and he still does this. Two weeks ago he was eating me out of house and home, now today I couldn't get him to eat a fraction of his usual amount.
I would just keep an eye on her symptoms and how much she weighs. You can put a call into the nurse at the doctors office too and tell them what is going on. They can advise you how long this should go on before you should be worried. I always call mine with questions like that.