I had the exact same problem with my son when he was 6 weeks old. It started when we upped his ozs from 3 to 4 per feeding. He did great for about a week and then began crying after only eating about 2 ozs and wouldn't finish his bottle for anything! We were afraid to stop to burp him, too! We thought maybe he had a dairy intolerance or perhaps we were using the wrong bottles...bad gas, etc. etc. I tried every bottle/nipple combo you can imagine and it didn't help. (For the mom who mentioned her baby downing the bottle in 5 mins I would suggest trying a slower nipple since they really should take at least 15 mins).
I got concerned that his daily intake was low for 3 or 4 days in a row, (~18 ozs instead of 20-25) and took him to see his Pediatrician. He suggested trying Zantac 2x a day and I was surprised, since I thought reflux meant that he needed to be spitting up, which he wasn't. I spent a while searching on the internet and found that he had most of the symptoms for "silent reflux". (
http://www.infantrefluxdisease.com was the most useful site I found). What happens is that the formula splashes up into the baby's esophagus when they eat because of an underdeveloped sphincter muscle in the stomach. Some MD's will recommend adding rice cereal to help "hold down" the food, but mine didn't advise this. I had pretty good success with Good Start formula because it digests quickly (emptying the stomach quickly can also help alleviate the amount of reflux). Some milk allergies will manifest themselves as reflux, so it's good to get some help from your MD to figure out the cause if the change in eating habits/fussiness persists.
The zantac definitely helped him eat more, but I felt that it was always a perpetual struggle to get him to eat enough. My advice is to get an MD opinion and hang in there! The good news is they DO outgrow it.