Yes, it is definitely possible you are adding muscle mass if you are new to your exercise plan. When my body is building muscle, it often retains fluid as part of the process.
Whenever I boosted my exercise, I usually had a scale stall to accompany it. Within the next couple weeks, I always had a nice loss.
Your plan looks reasonable, and healthy. Are your snacks all at once? Maybe spreading them out over the day would help.
And - definitely make sure you are weighing/measuring your food and writing down EVERYTHING accurately.
Jay's idea to drop down 100 cals is a good one.
Keep us posted on your progress