I can't pin point what the problem is here, I don't think any of us exactly can, we are not vets and we are not there.
But about your food question, you asked if you had fed them too much of the new stuff.
Keep in mind that the time to switch is over at LEAST a week, you can and should do more if your cats often have problems switching foods, or if they are going through some other stressed out period in their life.
Me, the first day, I'll give them 3 or 5 peices by hand as a treat to see if they even like the new food. And then come supper time I'll sprinkle maybe 5-8 peices in their bowl the first night.
And the following morning I'll move up to around 8-10 peices, the coming night will be about 12-15 peices. So on and so forth.
I like to keep a strict guard on how much I'm feeding. This way if something does go down and the vet asks "how much are you feeding" I don't have to estimate "ohh about a cup a day".
I've been feeding these 2 foods for about a week now, and today they are getting about 13 peices of their OLD food, and the rest is all new.
Of course, this can varie depending on the size of the cat, how much they eat in one sitting etc. Just a little guide line to how I keep on top of it.