Final Fantasy 7: Why does Tifa suck all of the sudden?

In FF7, Tifa has been my highest level character, consistently dealing the most damage, but when I went to the Sunken Gelnika sidequest, when I went to fight Reno and Rude, she mively declined her attacks. She used to deal 2000 damage average, and all of the sudden, she's dealing an average of 90. What happened? I didn't change her Materia, equipment, or Limit, I left her the same as i have for a while, and all of the sudden she just sucked. I'm really confused and upset, because Tifa was my best character, and I really don't want to put her out of my party. Please help!