If you didn't sheet out then it may well be the fin lifting the board out of the water. This can happen if the nose is raised too much, the angle of attach of the fin then makes it unstable and it flips the board up. You can try a smaller fin, or you sail higher and lower compared the wind direction, i.e. don't stay on a beam reach for long periods of time. This is how Formulas are sailed too, you need the large fin to get a proper lift when going very high or low, but it also creates a little too much lift when reaching.
So it is a matter of sailing style, and sailing spot, if you mainly want too sail back and forth on a reach, then yes try a little smaller fin. But don't put a too small fin on it, then the board will just stick to the water.