Could very well be - and that it wasn't lactose intolerance (which is very rare in babies) but a milk intolerance (much more common in babies) and as well, a soy problem. Gaviscon very often does cause constipation and firmer stools and so does soy formula - so if she's gone the other way then it could be an intolerance. Change of formula does often cause change in poops for a few days but if it's been over a week now I would expect it to be settled by now.
It is pretty rare for them to test babies, most of the time it's not worth putting them through the testing when clinical diagnosis can do the job just as well, and the tests are not 100% accurate. Can you go back and ask doc if you can try Nutramigen to see if she gets on any better with that? It's a milk based formula but the milk proteins are extensively hydrolysed and suitable for most allergic/intolerant babes. It's also lactose free.
HTH