Muscle does not weigh more than fat. One pound of muscle = one pound of fat. Muscle is denser than fat and therefore takes up less space. And actually, building some muscle means an increased metabolism and therefore more fat loss over time. The scale is pretty meaningless, actually. So, your weight could go up with muscle gain, but you'd lose inches and look much better. Then again, to gain a significant amount of muscle takes very specific nutrition and training, especially for women. I retain muscle well and dance a jig if I can gain 1-2 pounds of muscle during a "bulking" phase.
Additionally, when just starting out your body will repartition, meaning you can gain some muscle and lose fat at the same time (so a person will go down clothing sizes or lose inches, but weight about the same). This only happens in the beginning or when someone is deconditioned or using steroids, so generally you have to focus on either fat loss or muscle gain at a time (with periods of metabolic maintainance in between). My point is that muscle gain is something that has to have effort put into it...
Also, losing 10 lbs in one week is not healthy. Period. No ifs, ands or buts. Even water weight loss which is common in the beginning, is not 8-10 lbs worth. Be careful. If you eat too little you will damage your metabolism. In the first week I'd not be shocked at 3-4 lbs, but beyond that raises red flags in my mind. And after that, aim only for 1-2 lbs a week and no more. I'm not trying to be a downer... really, I'm not.
I'm a certified personal trainer (NASM-CPT and ISSA-CFT) and sports nutritionist. This is what I do for a living (I own a personal training and sports nutrition consulting business), so if you have questions.. PM me or post.