My DD2 had it and wasn't diagnosed till 8 months. She screamed constantly. She would scream for six to eight hours at a time, and I couldn't put her down. Id on't know how people do CIO because hearing my child cry that much was the most horrible thing I have ever dealt with as a parent.
Aside from screaming, she would gag and choke when she was lying down. You know how sometimes they gag in the first day or so after birth, getting the fluid up? That is what she did till nearly a year of age.
In fact, she wasn't gaining weight well, and so the doctor had a swallow study done. (Stupid friggen doctor, wouldn't address the screaming issue but dealt with the slow growth issue.) Anyway the swallow study showed some minor swallow issues, but more importantly, it showed everything refluxing almost immediately after ingestion. It would get almost to the back of her throat, but not quite high enough for her to spit it out.
We tried prevacid and zantac and ended up with tagamet as the most useful medication for her reflux. She still did reflux though, until around 18 months of age. She would drink something from a sippy cup, and walk away and spit some up and onto the floor.
My daughter's symptoms started within the first two weeks of her birth. If I could go back and do it differently, I would. I would have fought for her harder, switched doctors more (she saw 3 different doctors, I should have taken her to more than that.)