Nat is so right - women don't bulk without the help of hormones. To even begin to gain mass, a woman would not only have to lift heavy but also consume a lot of calories including a fair amount of carb. Low-carb dieting by itself would prevent any bulking from happening - and when I say bulking I mean "looking like Madonna", not "looking like Chyna".

What I do is alternate cardio days and lifting days. Another option is do cardio in the morning, lifting after work to be able to manage both and keep workouts short.