Megan's Secret

Megan had wealthy grandparents. They lived on The McCallum Estate, a vast property with beautiful grounds all around their large mansion. It had flower beds, trees, shrubs, and pools of water with fountains.
The McCallum mansion had a movie theater, swimming pool, fitness room, billiards room, library, a large ballroom, family room, and many large bedrooms and bathrooms. Megan always had fun when visiting her grandparent's mansion.
When Megan was growing up, she had a grandfather who loved her so much. She was very close to him. He rocked her when she was a baby, and helped her with her school work when she was older.
One day when she was seven, he discovered something about her that surprised him. He told Megan that she must never, ever tell anyone about what he found out about her. It must be kept a secret.
What could this secret be that no one could never know about her?
Also discover how Megan got involved with horses.