I would be very cautious before switching to soy. My niece (8 mos) was on the hypo allergnic formula since birth, as well. The ped suggested soy, without ever testing for milk allergies. She had such a severe allergic reaction my brother had to perform cpr on her. Thank God he recently finished his EMS training. She's back on the hypo allergenic formula.
My son had terrible spit-up (projectile) as an infant. I disagree with the mom that said that acid reflex is overly diagnosed. In fact, because their systems are so new and underdeveloped MOST infants have some form of reflex. Most peds will diagnose it, but most won't actually treat it with anything. My niece was diagnosed, but she still isn't on any meds for it.
I agree that you should go to a pediatric internal medicine specialist. Just make the appointment for one. If you need a referal, make the appt. and tell the dr's office to fax it over after the appt. has been made. If you don't need a referal, then get on it! Sometimes pediatricians aren't right. I had a "wonderful" experience with mine when I said my son was no longer talking and seemed distant. Know what he said to me? "Turn off the tv and talk to him." I never went back again. He was later diagnosed with mild form of Autism. That doctor's name is Stanley Rosenthal and he still practices here in Temple Terrace.
Parents have to learn to be their child's advocate. Call/go to an ER, or go to an children's after hours (they usually have x-rays and that sort of equipment, too) as suggested since it's the weekend. At the ER, they should do bloodwork, take tests, even give her a MRI or C-Scan if necessary. From there they should even suggest a good specialist. In the meantime, google her symptoms and you'll be surprised at the medical pages that pop up these days.
If it's a blockage, stomach, or intestine issue, it can be fixed, but how will you ever know if she's never tested? I wouldn't put it off a second longer.