A Ouija Board is not dangerous, it's a tool that's used correctly won't hurt you. Cast a circle and cleanse the board and area after use and you will be fine.
Regarding your question, it could be a few things, your window might be close but a breeze could still effect it, say if you were near a vent. But for argument sake let's say it was something, it might of been a deity since you were in circle. Easiest is to try try meditating, say hi, or ask a simple question like their name, then meditate and wait for a response, you might hear, feel, or see something, keep an open mind. If you're not good with meditation you could try skrying. Skrying can be done with a number of items, but the key is to clear your mind and gaze past your reflection. [If you're using a reflective surface] the pendulum is another item but if you can't keep your hand steady for long periods I wouldn't recommend it. There are many other options but those are probably your best bet.