Kim,
Muscle weighs more than fat so it could very well be muscle. Most olympic athletes have an "overweight" BMI or close to it because they have so much muscle mass even though their body fat percentage is incredibly low.
You could also be retaining water, especially if the pounds came on overnight. The best way to help with water retention is to drink more (counterintuitive, I know) and watch your sodium intake.
It could also be that you hit a plateau. Our bodies are designed to do lots of physical labor with very little food because that's what was needed for thousands of years of survival. It's just been in the last 100 years that food has been so plentiful and physical activity has become just a hobby. That means when you drop your food intake and increase your activity, your body will compensate to become more efficient - doing more and needing less fuel to do it with. Plateaus are usually conquered with patience and changing up your routine - what you eat, when and how you work out.
I would recommend you measure success with the measuring tape and by the fit of your clothes and rely less on the scale. If you do choose to weigh yourself, don't do it more than once a week and make sure it's at the same time of day, same day of the week and with the same amount of clothes - or lack thereof. All these factors can change your weight by pounds. You may also want to check your scale batteries to see if they need to be changed.
Good luck and keep up the good work,
S.