What Causes Fussiness And Less Appetite?

Our 5.5 month old daughter has been receiving prune juice in her bottles at every feeding. Every bottle is about 6-8 oz., sometimes more depending on time of day. She gets a 6 oz bottle at 8 am, a 6 oz bottle and 2 tbsp of baby food at 12pm, a 6-8 oz bottle at 4:30 or 5pm, and a 7-9 oz bottle at 8:30 or 9pm before bedtime. In the past 2 weeks, she has been more fussy during feeding times and won't take more than 4-5oz at times. Should we be giving her more food in addition to the bottle? We don't know what is wrong with her as we are first time parents.