Diagnosing pitching problems without actually seeing the delivery is not always accurate (not to mention sometimes dangerous), so take this with a grain of salt. But if your fastball has a dot in the middle of it, it sounds like you may be letting your wrist twist as you release the ball. I've seen this happen to pitchers when learning to throwing a slider (this problem is not only bad for the pitcher's arm/elbow in the long term, but also turns the slider into a kind of rolling---and very hittable---pitch). I suppose it could happen with a fastball, too (although not as easily as with a slider). If you think this is your problem, try concentrating on keeping your wrist firm and try to keep it behind the ball (coming straight through it) as you deliver the pitch.