Regarding the carbs, the answer must might be that whenever you eat a carb, you retain Water rather than actually gain fat. Carbs - even good carbs - make your body retain water. I assume with this that you're not making whole meals out of carbs, because if you are, it could actually be fat gain.
I know that whenever I've gone on low carb diets, all it took was a little bit of bread to make the scale jump, but it was never a long term gain if I just ate reasonably. So don't worry - it's probably just water. Just eat carbs in very small portions and you should be fine!