Rejecting breast or bottle, can be due to:
1) teething (this is temporary. Still nurse on-demand)
2) illness/being sick (still nurse on-demand so baby does not get dehydrated)
3) If solids is given..... ? Are you giving solids yet? Always... nurse or give bottle, FIRST and before any solids or other liquids. Or baby will be too full to nurse/take the bottle, after.
For the 1st year of life, breastmilk/Formula is a baby's primary source of nutrition. NOT solids and not other liquids. Solids, is not as nutritionally dense, as breastmilk/Formula
4) If your milk supply, is diminishing. (this is for if she is rejecting breast).
5) If the flow of the nipple (if giving a bottle) is too slow, per her age. As they get older, they get more efficient, at sucking/nursing. So change the nipple to one that is more appropriate for her age.
6) IF you 'replace' direct nursing at the breast (if baby is rejecting breast), and give a bottle instead... your milk supply, will decrease. Or, your body will adjust, per milk output from your breasts, to the frequency of how often you direct nurse.
7) the baby is rejecting breast (but this does not seem to be your case), because they are weaning, from breast... and prefers a bottle. (my son was like that).
8) ear ache
9) she just rather breastfeed.
But for the 1st year of life, breastmilk or Formula, is still very important and their main... source of nutrition. And nurse on-demand... day and night, and especially at growth-spurts. When you can.
My kids as babies, would temporarily reject breast, when teething. This may be also for a bottle.
Or perhaps, she is just not interested.
How many ounces, is she getting per day?
How often is she fed (the bottle)?
Now, are you using the bottle, predominantly?
Or breast?
Try to direct nurse, when you can, so your milk does not diminish and so that she is getting adequate intake.