I wonder if the flax is adding to the bad taste?

You might try a teaspoon of cocoa.. it adds 1 carb I think.
Depending on how strict you are there are syrups by Atkins and DiVinchi - I have not tried them yet though, I'm afraid of using too much.

The best tasting protein powder I've tried, by far, is Nitro Tech. Unfortunately it's expensive too There are three carbs and 22 grams of protein per scoop. It makes incredible mousse mixed into a thick pudding with cocoa and cream, then mixed up with whipped cream.

Good luck on finding a palatable drink!

Meg