It sounds like you already know how inaccurate weighing yourself after exercise is so I won't give you "the lecture".
Did you drink water during class? I chug lots of water when I spin because I sweat like a pig. A bottle of water weighs about a pound. Did you sweat more? Less? Was your shirt soaked? (mine is) Did you weigh with your sweaty clothes on? All these things show up as "weight" changes but they're not fat changes.
Spinning may be THE most calorie-burning workout, so keep it up -- I'm sure you're going to love the results!