My two month old daughter sometimes doesn't want to eat?

she eats about every three hours during the day and is sleeping abut 5 hours a time at night but sometimes she will only eat 3 oz out her usual 5 oz bottle is this normal she sometimes makes up for it in the evening by eating every two hours but sometimes she just doesn't want to eat and she fights sleep so bad! its impossible to get her to sleep at night sometimes! any suggestions? also she loved to sleep on her stomach is ok you think to let her do that as long as there is nothing that she can smother with she has acid reflux and im starting her to get used to her crib i have it elevated but she does so much better also im worried about choking as much as she spits up on her back?