First I was surprised that you had the pad inside your sleeping bag. Put it under the bag and inside the bivy and your bag will be up off the floor some. I have done a ton of research on bivies and used a fair number and like David said the WPB fabrics, other than eVent and Gore windstopper, are generally not breathable enough. Even eVent with a silnylon floor has problems from time to time.
You must have been in a really humid situation if the inside of the tent was dripping too. Bivies have the most trouble down in those temps just above freezing when the dew point is also very low. You can also try to suspend the top of the bivy with a bungee cord so that there is better air flow over the top of the sleeping bag.