You don't generally gain muscle that quickly, especially since you're female. Muscle building is a weeks- and months-long process. It's most likely water weight gain. Weight's not a static number, and it can fluctuate several pounds in a day! Losing weight isn't a linear process, and you can gain, lose, or stay the same week to week.
Have you updated your goals recently? As you lose weight, Sparkpeople doesn't adjust your calorie range automatically. Did you start a new fitness routine? I wouldn't worry overmuch over a one-week gain... if it continues, then you can start looking at tweaking something.