My DS2 fed like a dream until 10 weeks, but as he was my second I had already been through this with my first so knew what it was.
In both my sons cases it was reflux. DS1 would throw up violently and it never bothered him, however DS2 didn't, he had silent reflux and screamed the place down.
Symptoms were, that he had previously fed wonderfully, starting arching his back during feeds, reduced the amount he took, so instead of his usual 6oz feed he reduced this to 3oz. This would take ages to get down him. He would posset clear liquid with white flecks of milk in it hours after feeding. He would by smiling and happy and then wince like he had an awful taste in his mouth and also cough like he had choked.
We went to GP when he was around 15 weeks and were initially put on gaviscon which didn't work, we were referred to private paediatrician at BUPA hospital who diagnosed reflux (my wee one actually sicked up that clear liquid right in front of paed doc) it was £200 consultation.
He was prescribed Enfamil AR (anti reflux) which is a formula that thickens on contact with stomach acid (SMA staydown is the same thing - ie you mix it with COLD water so as not to activate the rice starch and you then heat the milk and they have it straight away)
Our paed doc didn't want to give him medication as he preferred the more natural route at the start. My son was on this from 17 weeks until he was 14 months, but it worked and was fab, it was on prescription so we never paid either.
You could try the SMA staydown, it is available in Boots etc but it is more expensive so if it does work maybe you could then go to the doctors and try to get the Enfamil AR.
Re the amount of formula, check the info on the one you are using. But it is only a guide some babies are more hungry and some eat less.