Well Ally, I def. agree with you on that last part-I think that she did have some magical encounter in her younger years, but I don't think she was jealous of Lily. In Philosopher's Stone, Petunia's sudden outburst was basically the first time she had ever mentioned her sister. I think that something happened when Lily was at school, and essentially scared Petunia into hating everything that had to do with magic. I think she's talking out of hatred rather than out of jealousy (in PS).
Congrats on your first thread!!