Ds was always a 'sicky' baby from day one but it is hard to know what is normal and what isn't when you are a first-time mum.
The frequency of vomiting gradually increased as time went on (i.e once every couple of day, then once a day, then after every other feed, then after every feed).
The 2 things that stick out in my mind that in hindsight, made it obvious that this wasn't just your average 'sicky' baby was firstly my mum saw him being sick after a feed when he was about 6 weeks old and said 'that isn't normal for a baby to be sick so violently (it was coming out of his nose aswell as mouth). Secondly, he was sick at my sisters about 2 hours after a feed and the whole feed came up - she said 'that isn't normal for so much milk to come back up that long after a feed'.
Also, DS never projectiled vomited - it was just very forceful and came gushing out but never really flew over the other side of the room.
In terms of diagnosis - I spoke to the HV when he was 6 weeks (when he was being sick once or twice a day) and she said it was probably reflux and he would grow out of it. I took him to the doctors that afternoon as he was violently sick again who diagnosed gastroentritis - and told me that diorrhea (sp?) would probably follow - whch of course it didn't. Vomiting got worse over the next few days so took him to a different GP who diagnosed reflux and gave us gaviscon which just made the vomit like porridge. That night, DS woke at about 2am, puked in his crib and there were reddy/brown bits in it which alarmed me so we took him straight to A&E. It was after 2 days in hospital that they eventually diagnosed it (I think because a nurse witnessed his vomiting and said how forceful it was).
BTW, they told me that the blood in his vomit was nothing really to be concerned about as it is just when people vomit frequently the stomach acid causes the top of the stomach to bleed (apparently common in bulimics aswell).
I know I have wittered on here but there may be something here that rings bells with you and helps you push for a diagnosis (or put your mind at rest that it isn't pyloric stenosis).
In terms of thriving, DS's weight gain was perfectly ok - I had him weighed at nearly 7 weeks and he was still on his original centile as he had been all along. It was only when they weighed him at A&E 5 days later that we realised he had lost over 1lb in less than a week so the weight drop was quite dramatic and only in the last week when he was vomiting after every feed.
As I said, I really wouldn't wish it on anyone because that whole period was really stressful - me and DH daren't leave the house in case DS vomited everywhere. The only time we would feel comfortable going anywhere was just after he had been sick but before his next feed
Apologies again for wittering but I hope this helps you.