Just to clarify, when you are "toning", no, you are not actually gaining muscle.
In order to truly gain muscle (of the sort that can show up on the scale as a gain) you have to eat at a calorie surplus (and yes, gain fat as well), and be training to gain muscle. It's nearly impossible to do this on a calorie deficit (what most of us trying to lose weight are eating at). The process of building muscle takes months or years... not hours, days, nor weeks.
You can't "pile on muscle". Although if you were, it would take up less room than the fat, and look better. ;) It's impossible to burn calories and not any fat at all, so rest easy.
This is also why the whole "lift light for lots of reps to avoid bulking up" thing is a total myth. Our bodies don't work that way. :)
Here's a great article that dispels many of the common myths about strength training.
Toning vs. Bulking Up: The Real Facts www.sparkpeople.com/resource/fitness_artic