Are you using Truvia as an artificial sweetener? If so, you may want to look at this article. It talks about the different brands of rebiana-based sweeteners and how they may be affected by additional ingredients. It also speaks to some of the science and marketing behind the products The comments were especially interesting to me, but keep in mind that they are purely subjective.
I used Truvia in the 2nd round of my Phase 2 and I was extremely happy with the taste and texture. I didn't notice any adverse effects on the weight loss and it made a nice change of pace from Splenda & Nutrasweet, both of which have a bit of an aftertaste to me. I haven't noticed this with Truvia so far.
Remember, you can use artificial sweeteners at any time during the HCG protocol. It's the carbs you want to stay away from, so read your labels and if you see any carbs, put it back on the shelf. Truvia can be found at most retail grocers, but if you are looking for the stevia extract itself, you may need to go to a whole foods or health food store.