How Do You Handle?

So Miss Sadie is 8 weeks old.  She is baby #5 and I have breastfed all my babies.  My mom has started questioning if she is getting enough. Snuck her a bottle of formula the other day (free sample she intercepted) and Sadie cried all day long.  I was/am not happy.  I have told her about how I feel but now I don't trust her.   Sadie has a very sensitive belly.  She has to be burped or will go crazy in the evening.  My mom believes she  is starving because of her crying.  How would you handle it?