I am also having the same problem right now. My Lo is 10 weeks tomorrow. She was taking 4 oz. every 2 1/2-3 hours. Then she decided she wanted six oz.at each feed for about a day and half. Now, she is taking 4-5 oz. (4 oz. most of the time) every 4-5 hours.
I was trying to follow a 4 hour easy and feed her every four. But, if she is not hungry, then she spits up most of the bottle. Plus, I can't figure out a 4 hour easy for her age.
So, needless to say, our Easy routine is gone for now.
We are going to her doctor tomorrow for her 2 month checkup. I plan to ask him what is best and how much she should eat.
I remember the amount my first child ate went in cycles. Meaning he would eat a lot a few days and then not so much the next. Getting shots did cause him to loose his appetite for a couple of days. But, he always caught himself back up.
If it doesn't change in a few days and you are still worried, you could always call your babie's doctor for reassurance.