Hi hun, Not too sure if they can suddenly develop reflux/colic but I know my two have had periods of what sounds like purple crying. Nothing wrong with them, just lots of crying!
Though diet did impact this crying. Are you having any gassy veg/spicy foods/citrus fruit or caffeine? I had to have a really plain diet and that really really helped with this crying period. It's only the past few weeks that the littlest hasn't been impacted by things like coffee (thank god!)
Also how many weeks is she? I found weeks 6,9,12 and 14 in both babies were the weeks they must've had some kind of growth spurt and cried a lot - nothing would soothe them other than the bath!
Otherwise cranial Oestopathy is meant to be good for babies who cry like this. I know you live north London and there's a drop in place which does free sessions on a Wednesday morning if you were ever interested!
I hope it eases for you, it's so upsetting when nothing soothes them.xxx