Eating less?

Carey
My LO usually eats 3 oz every 3-4 hours, but at 12 am, 3 am and 6 am she's only eaten 2oz, at 12 and 3, then 2.5 at 6. Is this normal for a 3 w old? She was 6 lbs 10 oz at birth,last Mon was 7.and I weighted myself then with her and she's 7.4 lbs now. But she won't eat more she just gets tired and takes the bottle out of her mouth.. she's formula fed only and having plenty of wet and poppy diapers.