Illegal Pick on a Screen play??

[J.Thurman]

played a game this weekend. Our league plays by NFL rules. We were in a double slot/double split formation. We brought our left Slot in motion across the formation. On the snap the playside slot blocked down on the first inside man while the motioning slot was to kick out the corner while the SE stepped back and took the screen pass. Well the Corner reacted and crossed the line. Just as the ball got to the SE the slot collided with the Corner. The pass was incomplete, but we were flagged 15 yards for it and it was a loss of down penalty. They called it an illegal pick. Can someone tell me if this was the right call?