I'm not going to give my lab partner the statistics from our experiment to include in her lab report
Is it because you two were once a couple?
Yes, she was always mean to me when we were together.
While she shouldn't have been cruel, that's in the past. You shouldn't go out of your way to harm her in the present.
You're right.  Who cares?
I'm glad you can put this behind you and act mature.  Let the past be the past and share the things you're expected to share with her.  This doesn't mean you need to be nice or anything, just honor your responsibilities!