My baby went through the same thing a couple of weeks ago (she was 4 months and one week then). She went from 5 oz to 2 oz!! I was really worried and asked a lot of people about this. I even went to the Dr and there was nothing wrong.
I later learnt that this is a phase which most formula fed babies go through. It is because they have had 'too much' milk in the past months and are using this time for their tiny organs to digest it all. So she was self regulating. It is less common with breast feeding.
My baby is being fed breast milk, but from a bottle, so I guess this phase has something to fo with bottle feeding.
The way I see it is that with bottle feeding, we dictate how much they drink and try to get them to finish everything in the bottle. Whilst those breastfeeding will stop when they are full. As parents, we forget to take into account that their appetites may vary from day to day.
Anyway, after nearly 2 full weeks of not eating much, my baby is back to normal and finishing off every drop like she used to! So don't worry, as long as the weight of your baby does not drop, then it's probably just the phase.
Hope that helps.