Idealfast shakes and diabetes?

I take glucophage twice daily to control my blood sugar but I'm noticing consistently high blood sugar readings while on the pre-op idealfast liquid diet. I would have thought my readings would go down, not up. I also noticed that the chocolate-flavored shakes are sweetened with sucralose AND sugar, could this be a factor? I've got another four weeks of this liquid diet to go and am a little concerned.