She can't affect things in the future? We can all affect things in the future. If I drop a ball, then at some point in the future it will hit the ground.
Besides which, Twilight has shown herself perfectly happy to create paradoxes. For example, she was told where to find the time travel spell by her future self, found the time travel spell, then used it to travel back in time and tell her past self where to find the spell. Paradox.
A paradox is only created in time travel if the cause of something happens after its effect. (As in the example above.) Though there's this whole light cone thing where if you travel not just back in time, but also far enough in space, you can do whatever the hell you like - even destroy the universe - without creating a paradox.