I wouldn't worry too much if you are happy you aren't doing a "well I've been to a spinning class therefore it won't hurt if I eat 3 Easter eggs" routinely . Easter eggs and AF will probably be playing their part but the body can behave a bit strangely when you start exercising regularly when previously you did little. It should settle down in a few more weeks.
Spinning will definitely burn lots of calories, tone your muscles, improve your fitness. It won't cause you to gain 2 stone of muscle by this time next year if you keep it up. Even body builders struggle to gain lbs of muscle and they have to eat lots and lots of calories in the form of protein and lift lots and lots of weight to do it. That's just the men, for women it is even harder. Your fat:muscle ratio should change for the better but your weight probably won't fluctuate (certainly not upwards) by much from spinning alone. Think about it, how many hugely muscular cyclists (outside a velodrome) do you see?