Hi guys when I take my chi out the the park to play with other dogs she cramps up and her whole body stiffens to the point she falls over. When I walk her on a lead she's fine and playing in the backyard is fine but when she does hard running chasing other dogs her whole body locks up. It's very distressing to see as she has that look where she's scared and frighten. After about 5 mins she's back to normal. I hate to have to limit her going out as she loves chasing her friends. What Is causing this and how can I prevent it?